test: co-locate test files with their source modules

[scottlovegrove]

Summary

• Moved all tests out of src/__tests__/ to sit next to the modules they cover (foo.ts → foo.test.ts).
• Relocated the shared test fixture to src/_fixtures/auth.ts.
• Rewrote relative import / vi.mock / dynamic import() specifiers for the new file depths.
• Updated tsconfig.json exclude (src/**/*.test.ts + src/_fixtures) so colocated tests and fixtures stay out of dist/, and refreshed the testing docs in CLAUDE.md.

No source logic changed — pure relocation. Git tracked every move as a rename so history is preserved.

Test plan

• npm run type-check clean
• npm run build clean, no *.test.* / _fixtures artifacts in dist/
• npm run test — 19 files, 166 tests pass
• npm run format clean

🤖 Generated with Claude Code

[doistbot]

This PR effectively reorganizes the test suite by co-locating test files with their corresponding source modules and updating the necessary configuration and imports to match. This restructuring is a great organizational improvement that will enhance maintainability and make navigating the codebase much easier. No issues were flagged during the review.

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