Add comprehensive tests for missing coverage areas

[devin-ai-integration]

Add comprehensive tests for missing coverage areas

Summary

This PR systematically addresses test coverage gaps across core FastAPI modules by adding comprehensive test suites for previously untested utility functions, compatibility layers, and concurrency utilities. The changes include:

• New test_utils.py: Complete coverage for 8 utility functions in fastapi/utils.py that had zero test coverage
• New test_compat_new.py: Comprehensive tests for Pydantic v1/v2 compatibility functions in fastapi/_compat.py
• New test_concurrency.py: Tests for concurrency utilities with various edge cases and error scenarios
• New test_background_tasks.py: Enhanced background task testing beyond existing coverage
• Enhanced test_fastapi_cli.py: Additional CLI test cases for better coverage
• Updated pyproject.toml: Added asyncio marker configuration for proper async test handling

Total additions: 664 lines of test code across 4 new test files covering critical but previously untested functionality.

Review & Testing Checklist for Human

• Run new test suite locally - Execute python -m pytest tests/test_utils.py tests/test_compat_new.py tests/test_concurrency.py tests/test_background_tasks.py -v to verify all tests pass and provide meaningful coverage
• Verify no regressions - Run the full existing test suite to ensure async marker configuration and new tests don't break existing functionality
• Manual verification of key utilities - Test a few core utility functions like deep_dict_update and is_body_allowed_for_status_code manually to ensure tests are checking correct behavior
• Review async test configuration - Verify that the pyproject.toml asyncio marker changes don't affect existing async tests negatively
• Code review test logic - Check test implementations for obvious gaps, incorrect assertions, or missing edge cases, especially in the complex Pydantic compatibility tests

Recommended test plan: Start by running the new tests individually, then run the full test suite. Pay special attention to any async-related test failures and verify the concurrency tests actually test the intended exception handling scenarios.

---

Diagram

graph TB
    subgraph "New Test Files"
        TU[tests/test_utils.py]:::major-edit
        TC[tests/test_compat_new.py]:::major-edit
        TCon[tests/test_concurrency.py]:::major-edit
        TB[tests/test_background_tasks.py]:::major-edit
    end
    
    subgraph "Enhanced Files"
        TCLI[tests/test_fastapi_cli.py]:::minor-edit
        PY[pyproject.toml]:::minor-edit
    end
    
    subgraph "Source Code Under Test"
        U[fastapi/utils.py]:::context
        COMPAT[fastapi/_compat.py]:::context
        CON[fastapi/concurrency.py]:::context
        BG[fastapi/background.py]:::context
        CLI[fastapi/cli.py]:::context
    end
    
    TU --> U
    TC --> COMPAT
    TCon --> CON
    TB --> BG
    TCLI --> CLI
    
    subgraph Legend
        L1[Major Edit]:::major-edit
        L2[Minor Edit]:::minor-edit
        L3[Context/No Edit]:::context
    end
    
    classDef major-edit fill:#90EE90
    classDef minor-edit fill:#87CEEB
    classDef context fill:#FFFFFF

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Notes

• Critical coverage gaps addressed: The fastapi/utils.py module contained 8 utility functions with zero test coverage, including complex functions like create_model_field and deep_dict_update that are used throughout the codebase
• Pydantic compatibility focus: The _compat.py module (665 lines) handles intricate Pydantic v1/v2 differences and had no dedicated tests - this was a significant risk area
• Async testing patterns: Used @pytest.mark.anyio following FastAPI's existing patterns rather than @pytest.mark.asyncio
• Test methodology: All tests follow existing FastAPI patterns using TestClient, pytest fixtures, and version-specific decorators for consistency
• Edge case emphasis: Tests specifically target error scenarios, exception handling, and boundary conditions that could cause runtime failures

Link to Devin run: https://app.devin.ai/sessions/dbd89234593447b88a819c77c57825b7

Requested by: @the-coding-gopher

[devin-ai-integration]

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