test(frontend): add Vitest + Testing Library and a frontend-test CI job

[lucastononro]

Summary

• frontend/package.json had no test runner (no jest/vitest/@testing-library/playwright) and no test script. Non-trivial client logic — the SSE dispatcher (useNotebookSSE.ts) and the fetch wrapper (api.ts) — shipped with zero coverage, so a single renamed event type or route path could silently break live updates with nothing to catch it.
• Adds Vitest + jsdom + React Testing Library, a test script, two real test suites (20 cases), and a frontend-test CI job.

Changes

• frontend/package.json: new test script (vitest run); devDependencies vitest, @vitejs/plugin-react, jsdom, @testing-library/react, @testing-library/jest-dom.
• frontend/vitest.config.ts (new): jsdom environment, @/* alias matching tsconfig.json, globals on, setup file.
• frontend/vitest.setup.ts (new): loads @testing-library/jest-dom/vitest matchers.
• frontend/src/lib/notebook/useNotebookSSE.test.ts (new, 9 tests): a fake EventSource (jsdom has none) drives the hook — verifies event routing per notebook.* type, per-notebook filtering of cell-lifecycle events, that kernel/structure events bypass the filter, malformed-payload tolerance, that the stream opens scoped to sessionId and is skipped when disabled, closes on unmount, and that handler updates take effect without re-opening the underlying EventSource.
• frontend/src/lib/api.test.ts (new, 11 tests): the fetchJSON-backed helpers (GET/POST bodies, conditional query-string building in listExperiments, the not-ok → Error path with status+text) and the raw-fetch FormData helpers (createExperiment's upload-failure path, attachData's webkitRelativePath-vs-plain-name fallback for folder uploads).
• .github/workflows/ci.yml: new frontend-test job — npm ci && npm test.

Test plan

Existing

• npx tsc --noEmit: clean.
• npx next lint: no warnings or errors.
• npx prettier --check 'src/**/*.{ts,tsx,css}': clean (ran prettier --write once to match house style before committing).
• npm run build: succeeds unchanged (7 static/dynamic routes generated).

New

• npm ci && npm test (the exact new CI command): 20 passed, 0 failed, 2 test files.

Caveats

• Scope is the two modules the issue calls out by name (SSE dispatcher + api.ts); AppContext.tsx, the lineage graph, and notebook cell components remain untested — good next targets but out of scope for this PR.
• No Playwright/e2e smoke test added (issue's "suggested fix" mentions it as an option); this PR sticks to unit/hook-level Vitest tests, which was enough to close the "zero tests" gap and land a real frontend-test CI gate.

Closes #114

🤖 Generated with Claude Code

Greptile Summary

This PR introduces the first testing infrastructure for the frontend: Vitest + jsdom + React Testing Library, a vitest.config.ts that mirrors the existing tsconfig alias, a vitest.setup.ts loader, and a frontend-test CI job that runs on every push alongside the existing lint and build jobs.

• api.test.ts (11 cases): validates GET/POST bodies, conditional query-string construction in listExperiments, the fetchJSON error-throw path, sendMessage conditional field inclusion, and both raw-fetch FormData helpers including the webkitRelativePath fallback for folder uploads.
• useNotebookSSE.test.ts (9 cases): a FakeEventSource drives the hook end-to-end — verifies session-scoped URL, disabled/null guards, per-notebook filtering, that kernel/structure/notebook.created events bypass the filter, filterRef and handlersRef live-update on re-render without re-opening the stream, malformed-payload tolerance, and EventSource.close() on unmount.
• .github/workflows/ci.yml: the new job is consistent with the existing frontend-lint and frontend-build jobs in structure, Node version, and npm cache configuration.

Confidence Score: 5/5

Safe to merge — adds test infrastructure and new test files only; no production code is modified.

All changes are additive (test files, config, CI job). The test assertions were verified against the actual implementation in api.ts and useNotebookSSE.ts — URLs, method names, error messages, and filter logic all match. The CI job structure is consistent with the existing frontend jobs.

No files require special attention.

Important Files Changed

Filename	Overview
.github/workflows/ci.yml	Adds frontend-test job that mirrors the structure of the existing frontend-lint and frontend-build jobs — correct working-directory, cache-dependency-path, and Node 20 setup.
frontend/vitest.config.ts	Configures jsdom environment, @/* alias matching tsconfig, globals, and the setup file. Config is minimal and correct.
frontend/package.json	Adds test script (vitest run) and five testing devDependencies; all version pins look current and consistent with package-lock.json.
frontend/src/lib/api.test.ts	11 tests covering fetchJSON-backed helpers (GET/POST bodies, conditional query-string, error throw) and raw-fetch FormData helpers (createExperiment upload failure, attachData webkitRelativePath fallback); test assertions match the actual implementation.
frontend/src/lib/notebook/useNotebookSSE.test.ts	9 tests with a well-structured FakeEventSource. Covers session-scoped URL, disabled/null guard, filter behaviour, handler-ref updates, malformed payload tolerance, and unmount cleanup. Three cell-lifecycle event types (cell.stream, cell.display, cell.error) remain untested.

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[greptile-apps]

Too many files changed for review. (122 files found, 100 file limit)

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[lucastononro]

Merged into integration branch staging-v0.0.5 (see the STAGING-v0.0.5.md ledger on that branch for merge order, Greptile follow-ups and test results). These changes will land on main via the staging merge — closing to clear the queue.