coverage: improve reporting and test coverage

[fe-dudu]

What changed

• Added codecov.yml to treat partial LCOV hits as hits.
• Added and updated tests to raise coverage in core analysis and webview utils.

Validation

• pnpm exec vitest run --coverage --coverage.reporter=text-summary --coverage.reporter=lcov
• pnpm exec biome lint src/core/analysis/__tests__/resolver.test.ts src/core/analysis/__tests__/queryKeyInternals.test.ts src/webview/utils/__tests__/detectTheme.test.ts src/webview/utils/__tests__/fileActionGroups.test.ts

[codecov-commenter]

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✅ All modified and coverable lines are covered by tests.
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see 23 files with indirect coverage changes

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