fix: resolve ChromaDB test mock setup and normalize_distance tests

[himanshu231204]

• Add mock_collection.count.return_value to mock_chromadb fixture to prevent TypeError when comparing MagicMock to int
• Fix TestChromaNormaliseDistance to use normalize_distance() from _scoring module instead of non-existent _normalise_distance method
• All 815 unit tests now pass

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Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Test update
• CI/CD update

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How Has This Been Tested?

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• Unit tests pass (uv run pytest)
• Linter passes (uv run ruff check .)
• Type checker passes (uv run mypy openagent_eval/)
• Manual testing performed

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• My code follows the project's coding standards
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

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