✅ Add a Jasmine to Vitest codemod and a migration guard

[thomas-lebeau]

Motivation

With Karma gone (#5142), specs still use the Jasmine API through a compatibility shim. This PR adds the groundwork to migrate them to the native Vitest API in small, independent PRs (one per helper or directory) without blocking other contributors: unmigrated specs keep working through the shim, and a file using import { describe, it, expect, vi } from 'vitest' gets the real Vitest functions (imports shadow the shim globals). Stacked on #5142.

Changes

Codemod — node scripts/test/jasmine-to-vitest.ts <files or directories> (scripts/test/lib/jasmineToVitest.ts)

• TypeScript-AST based, editing the source text in place (formatting and comments preserved), then Prettier.
• Converts: imports (describe, it, expect, hooks, vi, Mock/Mocked types), jasmine.createSpy → vi.fn, spyOn/spyOnProperty → vi.spyOn (bare spies are stubbed like Jasmine does: .mockReturnValue(undefined as never); .and.callThrough() dropped), .and.* strategies, .calls.* (count, argsFor, allArgs, reset, mostRecent().args, first().args, optional chaining kept), matchers (toHaveBeenCalledOnceWith → toHaveBeenCalledExactlyOnceWith, toBeTrue/False, toHaveSize → toHaveLength, withContext), asymmetric matchers, expectAsync resolves/rejects, done callbacks → Promise, pending() → ctx.skip(), fail() → expect.unreachable(), fit/fdescribe/xit/xdescribe, jasmine.Spy/SpyObj/Func types.
• Anything it can't convert safely (e.g. jasmine.createSpyObj, spy.calls.all(), jasmine.clock(), custom asymmetric matchers) is left untouched and reported with file:line.
• Deterministic: on a merge conflict, take the file from main and re-run it.
• Unit tests: scripts/test/lib/jasmineToVitest.spec.ts (40 cases, run by yarn test:script).

Migration guard — VITEST_API_SPEC_FILES in eslint.config.ts: migrated paths are added there, which forbids Jasmine globals (jasmine, spyOn, fail, pending, …) and requires importing test functions from 'vitest'. Empty for now.

Other

• vitest declared as a devDependency of every workspace with specs (needed by import-x/no-extraneous-dependencies once specs import it).
• AGENTS.md: write new specs with the Vitest API.
• Removed the unused mockContextManager test helper.

No spec is converted in this PR. Trial run: on browser-rum-slim, browser-worker, browser-plugin-wasm and js-core (16 specs), 15 pass right after the codemod; the last one uses the reported jasmine.createSpyObj. On browser-logs, failures come from helpers still returning Jasmine spies (replaceMockableWithSpy), to be migrated together with their consumers.

Next PRs: shared helpers (mockClock, then each spy-returning helper with its consumers), then codemod sweeps per directory.

⚠️ Written with an AI agent (Claude Code).

Test instructions

yarn
node --test scripts/test/lib/jasmineToVitest.spec.ts
# trial conversion (don't commit):
node scripts/test/jasmine-to-vitest.ts packages/js-core && yarn test:unit packages/js-core
git checkout -- packages

Checklist

• Tested locally
• Tested on staging
• Added unit tests for this change.
• Added e2e/integration tests for this change.
• Updated documentation and/or relevant AGENTS.md file

[cit-pr-commenter-54b7da]

Bundles Sizes Evolution

📦 Bundle Name	Base Size	Local Size	𝚫	𝚫%	Status
Rum Salesforce	N/A	188.06 KiB	N/A	N/A	N/A
Rum	188.04 KiB	188.04 KiB	0 B	0.00%	✅
Rum-salesforce Profiler	N/A	8.35 KiB	N/A	N/A	N/A
Rum Profiler	8.35 KiB	8.35 KiB	0 B	0.00%	✅
Rum-salesforce Recorder	N/A	32.04 KiB	N/A	N/A	N/A
Rum Recorder	32.04 KiB	32.04 KiB	0 B	0.00%	✅
Logs	59.44 KiB	59.44 KiB	0 B	0.00%	✅
Rum Slim	145.83 KiB	145.83 KiB	0 B	0.00%	✅
Worker	22.96 KiB	22.96 KiB	0 B	0.00%	✅
Rum Shopify	N/A	218.48 KiB	N/A	N/A	N/A
Rum-shopify Profiler	N/A	8.35 KiB	N/A	N/A	N/A
Rum-shopify Recorder	N/A	4.85 KiB	N/A	N/A	N/A

[datadog-datadog-us1-prod]

✅ All CI checks and tests passed.

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 78.23% (-0.03%)

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