DagBag: use Path.relative_to for consistent cross-platform behavior

[Dev-iL]

closes: #45172

Verified on linux via the unit tests on on win11 by running the script below:

# test_windows_path_fix.py
"""
Test script to verify the path normalization fix for Windows.
This tests the core logic without requiring a full Airflow installation.
"""
from pathlib import Path


def test_path_normalization():
    """
    Test that the fix correctly normalizes paths with mixed separators.
    
    This replicates the fix in airflow/dag_processing/dagbag.py:
        normalized_filepath = Path(filepath).as_posix()
        normalized_dags_folder = Path(settings.DAGS_FOLDER).as_posix()
        relative_file = normalized_filepath.replace(normalized_dags_folder, "")
    """
    test_cases = [
        # (dags_folder, filepath, expected_relative)
        # Windows backslash paths
        ("C:\\Users\\user\\dags", "C:\\Users\\user\\dags\\my_dag.py", "/my_dag.py"),
        # Mixed separators (the bug scenario from the issue)
        ("C:/Users/user/dags", "C:\\Users\\user\\dags\\my_dag.py", "/my_dag.py"),
        ("C:\\Users\\user/dags", "C:/Users/user/dags/my_dag.py", "/my_dag.py"),
        # Nested paths with mixed separators
        ("C:/Users/user/dags", "C:\\Users\\user\\dags\\subdir\\my_dag.py", "/subdir/my_dag.py"),
        # Forward slashes only (should still work)
        ("C:/Users/user/dags", "C:/Users/user/dags/my_dag.py", "/my_dag.py"),
    ]
    
    print("Testing path normalization fix...")
    print("-" * 60)
    
    all_passed = True
    for dags_folder, filepath, expected in test_cases:
        # This is the fix from dagbag.py
        normalized_filepath = Path(filepath).as_posix()
        normalized_dags_folder = Path(dags_folder).as_posix()
        relative_file = normalized_filepath.replace(normalized_dags_folder, "")
        
        passed = relative_file == expected
        status = "✓ PASS" if passed else "✗ FAIL"
        
        print(f"\n{status}")
        print(f"  DAGS_FOLDER: {dags_folder}")
        print(f"  filepath:    {filepath}")
        print(f"  Result:      {relative_file}")
        if not passed:
            print(f"  Expected:    {expected}")
            all_passed = False
    
    print("\n" + "-" * 60)
    
    # Also demonstrate what the OLD buggy code would have produced
    print("\nDemonstrating the OLD buggy behavior (without fix):")
    dags_folder = "C:/Users/user/dags"  # Forward slashes
    filepath = "C:\\Users\\user\\dags\\my_dag.py"  # Backslashes
    
    # Old code: simple string replace without normalization
    old_result = filepath.replace(dags_folder, "")
    print(f"  DAGS_FOLDER: {dags_folder}")
    print(f"  filepath:    {filepath}")
    print(f"  Old result:  {old_result}  <- BUG: absolute path returned!")
    
    # New code: normalize first
    new_result = Path(filepath).as_posix().replace(Path(dags_folder).as_posix(), "")
    print(f"  New result:  {new_result}  <- FIXED: relative path returned!")
    
    print("\n" + "=" * 60)
    if all_passed:
        print("✅ All tests passed! The fix works correctly on Windows.")
    else:
        print("❌ Some tests failed.")
    
    return all_passed


if __name__ == "__main__":
    success = test_path_normalization()
    exit(0 if success else 1)

---

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[potiuk]

Fantastic !

[potiuk]

Rebased - the static error was caused by the breaing FastAPI change.