[User Storage Service] browser client, React hooks, injection rendering (elastic#268312)

Epic: elastic/kibana-team#2769

## Summary

Adds the browser foundation for the Core User Storage service (Step 2 of
the project; the server-side foundation landed in elastic#256912).

Three pieces:

**A. Server-rendered injection.** 
- RenderingService` pre-fetches the current user's stored values per
request via `userStorage.asScoped(request).getAll()` and inlines them
under a new `userStorage.values` field on `<kbn-injected-metadata>`.
- Bounded by a 50ms timeout (mirroring the existing `clusterInfo`
pattern at `rendering_service.tsx:234`)
- skipped for anonymous pages and when `asScoped` returns `null` (no
`profile_uid`).

**B. Browser client.**
  - Three new packages in `src/core/packages/user-storage/`:
1. `@kbn/core-user-storage-browser` (public API: `IUserStorageClient`,
`UserStorageProvider`, `useUserStorage`, `useUserStorageClient`)
2. `@kbn/core-user-storage-browser-internal` (`UserStorageService`,
`UserStorageClient`, `UserStorageApi`)
    3. `@kbn/core-user-storage-browser-mocks`.
- The client seeds an in-memory cache from injected metadata at boot, so
`get(key)` is synchronous at first paint. `set`/`remove` write through
HTTP and refresh the cache on success.
  - Architectural diagram
<img width="1295" height="631" alt="image"
src="https://github.com/user-attachments/assets/0b3e9113-9ef1-47d7-8f7d-37bf8730e432"
/>

**C. React hook + provider.**
- `useUserStorage(key, default)` returns `[value, setter]` with an
Observable-backed subscription, and throws a helpful error when used
outside `<UserStorageProvider>`.
- `useUserStorageClient()` is the escape hatch for consumers that depend
on `remove`, `getUpdate$`, and `getUpdateErrors$`.

## Checklist

Check the PR satisfies following conditions. 

Reviewers should verify this PR satisfies this list as well.

- ~[ ] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/src/platform/packages/shared/kbn-i18n/README.md)~
- [x]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- ~[ ] If a plugin configuration key changed, check if it needs to be
allowlisted in the cloud and added to the [docker
list](https://github.com/elastic/kibana/blob/main/src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker)~
- ~[ ] This was checked for breaking HTTP API changes, and any breaking
changes have been approved by the breaking-change committee. The
`release_note:breaking` label should be applied in these situations.~
- ~[ ] [Flaky Test
Runner](https://ci-stats.kibana.dev/trigger_flaky_test_runner/1) was
used on any tests changed~
- [x] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)
- [x] Review the [backport
guidelines](https://docs.google.com/document/d/1VyN5k91e5OVumlc0Gb9RPa3h1ewuPE705nRtioPiTvY/edit?usp=sharing)
and apply applicable `backport:*` labels.

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>
Co-authored-by: Anton Dosov <anton.dosov@elastic.co>

Author: 3 people

.github/CODEOWNERS

@@ -349,11 +349,13 @@ src/core/packages/user-profile/server-mocks @elastic/kibana-core
 src/core/packages/user-settings/server @elastic/kibana-security
 src/core/packages/user-settings/server-internal @elastic/kibana-security
 src/core/packages/user-settings/server-mocks @elastic/kibana-security
+src/core/packages/user-storage/browser @elastic/appex-sharedux
+src/core/packages/user-storage/browser-internal @elastic/appex-sharedux
+src/core/packages/user-storage/browser-mocks @elastic/appex-sharedux
 src/core/packages/user-storage/common @elastic/appex-sharedux
 src/core/packages/user-storage/server @elastic/appex-sharedux
 src/core/packages/user-storage/server-internal @elastic/appex-sharedux
 src/core/packages/user-storage/server-mocks @elastic/appex-sharedux
-src/core/packages/user-storage/test/plugins/user_storage_test @elastic/appex-sharedux
 src/core/test-helpers/kbn-server @elastic/kibana-core
 src/core/test-helpers/model-versions @elastic/kibana-core
 src/platform/kbn-ui/chrome-layout @elastic/appex-sharedux
@@ -927,6 +929,7 @@ src/platform/test/plugin_functional/plugins/ui_settings_plugin @elastic/kibana-c
 src/platform/test/plugin_functional/plugins/usage_collection @elastic/kibana-core
 src/platform/test/server_integration/plugins/status_plugin_a @elastic/kibana-core
 src/platform/test/server_integration/plugins/status_plugin_b @elastic/kibana-core
+src/platform/test/user_storage/plugins/user_storage_test @elastic/appex-sharedux
 src/setup_node_env @elastic/kibana-operations
 x-pack/examples/alerting_example @elastic/response-ops
 x-pack/examples/embedded_lens_example @elastic/kibana-visualizations

package.json

@@ -533,6 +533,8 @@
     "@kbn/core-user-settings-server": "link:src/core/packages/user-settings/server",
     "@kbn/core-user-settings-server-internal": "link:src/core/packages/user-settings/server-internal",
     "@kbn/core-user-settings-server-mocks": "link:src/core/packages/user-settings/server-mocks",
+    "@kbn/core-user-storage-browser": "link:src/core/packages/user-storage/browser",
+    "@kbn/core-user-storage-browser-internal": "link:src/core/packages/user-storage/browser-internal",
     "@kbn/core-user-storage-common": "link:src/core/packages/user-storage/common",
     "@kbn/core-user-storage-server": "link:src/core/packages/user-storage/server",
     "@kbn/core-user-storage-server-internal": "link:src/core/packages/user-storage/server-internal",
@@ -1245,7 +1247,7 @@
     "@kbn/user-profile-components": "link:src/platform/packages/shared/kbn-user-profile-components",
     "@kbn/user-profile-examples-plugin": "link:examples/user_profile_examples",
     "@kbn/user-profiles-consumer-plugin": "link:x-pack/platform/test/security_api_integration/plugins/user_profiles_consumer",
-    "@kbn/user-storage-test-plugin": "link:src/core/packages/user-storage/test/plugins/user_storage_test",
+    "@kbn/user-storage-test-plugin": "link:src/platform/test/user_storage/plugins/user_storage_test",
     "@kbn/utility-types": "link:src/platform/packages/shared/kbn-utility-types",
     "@kbn/utility-types-jest": "link:src/platform/packages/shared/kbn-utility-types-jest",
     "@kbn/utils": "link:src/platform/packages/shared/kbn-utils",
@@ -1710,6 +1712,7 @@
     "@kbn/core-ui-settings-server-mocks": "link:src/core/packages/ui-settings/server-mocks",
     "@kbn/core-usage-data-server-mocks": "link:src/core/packages/usage-data/server-mocks",
     "@kbn/core-user-activity-server-mocks": "link:src/core/packages/user-activity/server-mocks",
+    "@kbn/core-user-storage-browser-mocks": "link:src/core/packages/user-storage/browser-mocks",
     "@kbn/cypress-config": "link:src/platform/packages/shared/kbn-cypress-config",
     "@kbn/cypress-test-helper": "link:src/platform/packages/shared/kbn-cypress-test-helper",
     "@kbn/dependency-ownership": "link:packages/kbn-dependency-ownership",

packages/kbn-optimizer/limits.yml

@@ -29,7 +29,7 @@ pageLoadAssetSize:
   contentConnectors: 33014
   contentManagement: 8350
   controls: 10300
-  core: 547151
+  core: 548600
   cps: 9209
   crossClusterReplication: 12662
   customIntegrations: 11715

src/core/packages/injected-metadata/browser-internal/src/injected_metadata_service.test.ts

@@ -195,3 +195,17 @@ describe('setup.getFeatureFlags()', () => {
     expect(contract.getFeatureFlags()).toBeUndefined();
   });
 });
+
+describe('setup.getUserStorage()', () => {
+  it('returns injectedMetadata.userStorage', () => {
+    const injectedMetadata = new InjectedMetadataService({
+      injectedMetadata: {
+        userStorage: { values: { 'navigation:layout': { hidden: ['discover'] } } },
+      },
+    } as unknown as InjectedMetadataParams);
+
+    expect(injectedMetadata.setup().getUserStorage()).toEqual({
+      values: { 'navigation:layout': { hidden: ['discover'] } },
+    });
+  });
+});

src/core/packages/injected-metadata/browser-internal/src/injected_metadata_service.ts

@@ -99,6 +99,10 @@ export class InjectedMetadataService {
       getFeatureFlags: () => {
         return this.state.featureFlags;
       },
+
+      getUserStorage: () => {
+        return this.state.userStorage;
+      },
     };
   }
 }

src/core/packages/injected-metadata/browser-internal/src/types.ts

@@ -64,6 +64,9 @@ export interface InternalInjectedMetadataSetup {
         initialFeatureFlags: Record<string, unknown>;
       }
     | undefined;
+  getUserStorage: () => {
+    values: Record<string, unknown>;
+  };
 }
 
 /** @internal */

src/core/packages/injected-metadata/browser-mocks/src/injected_metadata_service.mock.ts

@@ -54,6 +54,7 @@ const createSetupContractMock = () => {
     getKibanaBuildNumber: jest.fn(),
     getCustomBranding: jest.fn(),
     getFeatureFlags: jest.fn(),
+    getUserStorage: jest.fn().mockReturnValue({ values: {} }),
   });
 
   return setupContract;

src/core/packages/injected-metadata/common-internal/src/types.ts

@@ -93,4 +93,7 @@ export interface InjectedMetadata {
     };
   };
   customBranding: Pick<CustomBranding, 'logo' | 'customizedLogo' | 'pageTitle'>;
+  userStorage: {
+    values: Record<string, unknown>;
+  };
 }

src/core/packages/lifecycle/browser-mocks/moon.yml

@@ -37,6 +37,7 @@ dependsOn:
   - '@kbn/core-rendering-browser-mocks'
   - '@kbn/core-pricing-browser-mocks'
   - '@kbn/core-di-mocks'
+  - '@kbn/core-user-storage-browser-mocks'
   - '@kbn/lazy-object'
 tags:
   - shared-browser

src/core/packages/lifecycle/browser-mocks/src/core_setup.mock.ts

@@ -24,6 +24,7 @@ import { userProfileServiceMock } from '@kbn/core-user-profile-browser-mocks';
 import { createCoreStartMock } from './core_start.mock';
 import { coreFeatureFlagsMock } from '@kbn/core-feature-flags-browser-mocks';
 import { chromeServiceMock } from '@kbn/core-chrome-browser-mocks';
+import { userStorageServiceMock } from '@kbn/core-user-storage-browser-mocks';
 import { lazyObject } from '@kbn/lazy-object';
 
 export function createCoreSetupMock({
@@ -56,6 +57,7 @@ export function createCoreSetupMock({
     theme: themeServiceMock.createSetupContract(),
     security: securityServiceMock.createSetup(),
     userProfile: userProfileServiceMock.createSetup(),
+    userStorage: userStorageServiceMock.createSetupContract(),
     plugins: lazyObject({
       onSetup: jest.fn(),
       onStart: jest.fn(),