Fix git status manager test mock

[parsakhaz]

Summary

• mock the current getSessionComparisonBranch dependency in GitStatusManager tests
• restores validation coverage for git status state scenarios

Testing

• pnpm --filter main exec vitest run src/services/tests/gitStatusManager.test.ts
• pnpm --filter main typecheck
• pnpm --filter main exec eslint src/services/tests/gitStatusManager.test.ts --quiet
• pre-commit: pnpm typecheck && pnpm lint

Note: local validation emitted existing Node engine warnings because this shell is Node v20.19.3 and repo declares >=22.14.0.