[core][http] Add spaceId to KibanaRequestState and KibanaRequest (US-001) (#269158)

## Summary

Phase 1, US-001 of
[elastic/kibana-team#3204](elastic/kibana-team#3204):
_First-Class Space ID on Core Request_.

Adds `spaceId: string` as a first-class property on `KibanaRequest` and
`KibanaRequestState` (server side), with the backing implementation in
`CoreKibanaRequest`.

### What this PR does

- Adds `spaceId: string` to `KibanaRequestState` (the per-request Hapi
app state).
- Adds `readonly spaceId: string` to the `KibanaRequest` public
interface with JSDoc.
- Implements `spaceId` on `CoreKibanaRequest`, reading from
`appState.spaceId` and defaulting to `'default'`. This means the
property is **always populated** — never `undefined` — even before
US-002 writes the resolved space into the app state.
- Updates the `createKibanaRequest` mock fixture default to include
`spaceId: 'default'` so existing tests compile without changes.

### What this PR does NOT do

- No space extraction from the URL yet (that is US-002, which will write
to `app.spaceId` in the `onRequest` handler in `http_server.ts`).
- No changes to the Spaces plugin, `@kbn/spaces-utils`, or any
downstream consumers.

### Checklist

- [x] `node scripts/type_check --project
src/core/packages/http/server/tsconfig.json` ✅
- [x] `node scripts/type_check --project
src/core/packages/http/router-server-internal/tsconfig.json` ✅
- [x] `node scripts/type_check --project
src/core/packages/http/router-server-mocks/tsconfig.json` ✅
- [x] `node scripts/type_check --project
src/core/packages/http/server-internal/tsconfig.json` ✅
- [x] `node scripts/check_changes.ts` ✅

Made with [Cursor](https://cursor.com)

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: gsoldevila

src/core/packages/http/router-server-internal/src/request.ts

@@ -154,6 +154,8 @@ export class CoreKibanaRequest<
   public readonly authzResult?: Record<string, boolean>;
   /** {@inheritDoc KibanaRequest.timing} */
   public readonly serverTiming: RequestTiming;
+  /** {@inheritDoc KibanaRequest.spaceId} */
+  public readonly spaceId: string;
 
   /** @internal */
   protected readonly [requestSymbol]!: Request;
@@ -182,6 +184,7 @@ export class CoreKibanaRequest<
     this.uuid = appState?.requestUuid ?? uuidv4();
     this.rewrittenUrl = appState?.rewrittenUrl;
     this.authzResult = appState?.authzResult;
+    this.spaceId = appState?.spaceId ?? 'default';
     this.serverTiming = new RequestTimingImpl(appState?.timingState ?? { events: [] });
     this.injectHostInfo(request);
 

src/core/packages/http/server/src/router/request.ts

@@ -63,6 +63,8 @@ export interface KibanaRequestState extends RequestApplicationState {
   startTime: number;
   redactedSessionId?: string;
   timingState?: RequestTimingState;
+  /** The resolved Kibana space ID for this request. Set by Core's onRequest handler; defaults to `'default'` when absent. */
+  spaceId?: string;
 }
 
 /**
@@ -245,6 +247,15 @@ export interface KibanaRequest<
    * Only available during development.
    */
   readonly serverTiming: RequestTiming;
+
+  /**
+   * The resolved Kibana space ID for this request.
+   *
+   * @remarks
+   * Always populated. Defaults to `'default'` for requests that do not target an explicit space
+   * (i.e. requests whose URL does not contain a `/s/{spaceId}` prefix).
+   */
+  readonly spaceId: string;
 }
 
 /**