test: add shared sanitized GitHub API fixtures - #245

Summary

Closes #232 by introducing shared, privacy-sanitized GitHub API v3 JSON fixtures under fixtures/ for backend tests (pytest and unittest) and frontend mock API / Jest tests.

Changes

• Added 16 shared JSON fixtures under fixtures/ covering users (4), repositories (6), languages (3), and public events (3), including standard, empty, fork-only, no-bio, large-collection (100 repos / 301 profile), and multi-page pagination scenarios (12 real sanitized responses, 1 derived 4-event subset, 3 deterministic synthetic datasets).
• Documented fixture provenance, allowlist sanitization rules (email: null, private: false, public: true, removed node_id, payload.commits, and verbose hypermedia fields), and cross-stack usage in fixtures/README.md.
• Added path-validated backend/tests/fixture_loader.py, session fixture fixture_loader in backend/tests/conftest.py, and backend/tests/test_fixtures_sanitization.py enforcing recursive node_id, email/token, event/privacy, language byte, and edge-case invariants.
• Updated backend tests (test_github_routes.py, test_github_service.py, test_github_pagination.py, test_skill_service.py) to consume shared fixtures while preserving exact route assertions and exercising languages_empty.json, languages_single.json, and languages_multi.json.
• Integrated shared fixtures into frontend mock data (frontend/mocks/fixtures/users.ts, repositories.ts, skills.ts) and frontend/mocks/handlers.ts (aligning /api/github/users/:username/repositories with backend RepositoryListSerializer metadata while retaining legacy octocat / count assertions), and added frontend/__tests__/githubFixtures.test.ts.
• Updated docs/mock-api.md and CHANGELOG.md (Unreleased).

Verification

• Targeted backend pytest (test_fixtures_sanitization.py, test_github_routes.py, test_github_service.py, test_github_pagination.py, test_skill_service.py): 62 passed (exit 0).
• Backend unittest (python -m unittest discover -s tests -p "test_skill_service.py" -v from backend/): 12 passed (exit 0).
• New frontend fixture Jest suite (npx jest __tests__/githubFixtures.test.ts): 11 passed (exit 0).
• Existing mock API direct handler tests (handleMockApiRequest in __tests__/mockApi.test.ts): 10/10 passed.
• Ruff (ruff check and ruff format --check on fixture_loader.py, test_fixtures_sanitization.py, conftest.py, test_github_routes.py, test_github_service.py, test_skill_service.py): passed (exit 0).
• Prettier (npx prettier --check across all 24 Issue Create sanitized GitHub API fixtures for tests #232 JSON, Markdown, and TypeScript files): passed (exit 0).
• git diff --check: passed (exit 0).
• npx tsc --noEmit: 0 errors in Issue Create sanitized GitHub API fixtures for tests #232 files.
• (Note: mypy is not installed in the local Python environment, and npm run build was not executed directly after the final edits on the workspace; 0 application files under app/ were modified.)

Existing baseline limitations

• Full repository verification is not green on upstream base 37192a9:
  • Full backend pytest retains 1 failure reproduced on 37192a9 (tests/test_config_and_utils_typing.py::test_validation_utility_types rejecting @torvalds; 260 passed on the branch vs 243 passed on 37192a9) and exits with Windows teardown status -1073741819.
  • backend/tests/test_github_pagination.py retains pre-existing Ruff diagnostics on untouched lines (17: unused F401 Settings, 201: UP017 timezone.utc, 228: line wrap; repo-wide ruff format --check . fails on 25 upstream files), while all +43 added lines pass cleanly.
  • Full frontend Jest retains 27 failing suites and 6 failing jsdom Response-related E2E tests inside mockApi.test.ts reproduced identically on 37192a9 (16 passed, 27 failed suites; 223 passed, 6 failed tests vs base 15 passed, 27 failed; 212 passed, 6 failed due to upstream [email protected] vs [email protected], JSX syntax in .test.ts files, and missing Response in jsdom).
  • npx tsc --noEmit retains 23 pre-existing diagnostic lines in 4 .test.ts files containing JSX (activitySummary.test.ts, preferences.test.ts, profileHealth.test.ts, skillTimeline.test.ts).
  • npm run lint fails identically on 37192a9 (ESLint 10.14.0 vs eslint-plugin-react: contextOrFilename.getFilename is not a function), and npm run format:check referenced by CI is missing from frontend/package.json.

Testing

• Fixture & cross-stack tests: backend/tests/test_fixtures_sanitization.py checks JSON validity, recursive node_id removal, absence of email/token/payload.commits, repository private: false, event public: true, language integer invariants, and edge-case counts/fields. Shared fixtures are integrated and verified across test_github_routes.py, test_github_service.py, test_github_pagination.py, test_skill_service.py, and frontend/__tests__/githubFixtures.test.ts (including deterministic fixedNow activity summary classification for events_active.json, events_occasional.json, and events_empty.json, @octocat-real consistency, and /repositories pagination metadata).
• Upstream baseline reproduction: Backend pytest, ruff, frontend jest --ci, tsc --noEmit, and npm run lint were executed on an isolated detached worktree of 37192a9 to confirm every failing full-suite check pre-exists upstream.

Scope

No production application code (backend/app/, frontend/app/, frontend/components/, frontend/utils/) was modified. Changes are strictly limited to shared fixtures (fixtures/), test helpers and suites (backend/tests/, frontend/__tests__/), frontend mocks (frontend/mocks/), and documentation (fixtures/README.md, docs/mock-api.md, CHANGELOG.md).