Fix warning message for missing METADATA file

Fixes #12446

Update the warning message for missing `METADATA` file in dist-info directories.

* Update the warning message in `src/pip/_internal/metadata/importlib/_compat.py` to differentiate between a missing `METADATA` file and other metadata issues.
* Add a check for the existence of the `METADATA` file in the `get_dist_canonical_name` function.
* Add a test case in `tests/functional/test_list.py` to verify the new warning message for a missing `METADATA` file.

---

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src/pip/_internal/metadata/importlib/_compat.py

@@ -81,5 +81,8 @@ def get_dist_canonical_name(dist: importlib.metadata.Distribution) -> Normalized
 
     name = cast(Any, dist).name
     if not isinstance(name, str):
+        info_location = get_info_location(dist)
+        if info_location and not info_location.joinpath("METADATA").exists():
+            raise BadMetadata(dist, reason="missing `METADATA` file")
         raise BadMetadata(dist, reason="invalid metadata entry 'name'")
     return canonicalize_name(name)

tests/functional/test_list.py

@@ -751,3 +751,19 @@ def test_list_pep610_editable(script: PipTestEnvironment) -> None:
             break
     else:
         pytest.fail("package 'testpkg' not found in pip list result")
+
+
+def test_list_missing_metadata_warning(script: PipTestEnvironment) -> None:
+    """
+    Test that a warning is shown when a dist-info directory is missing the METADATA file.
+    """
+    # Create a test package and create .dist-info dir without METADATA file
+    pkg_path = create_test_package_with_setup(script, name="testpkg", version="1.0")
+    dist_info_path = pkg_path / "testpkg-1.0.dist-info"
+    dist_info_path.mkdir()
+    dist_info_path.joinpath("RECORD").write_text("")
+
+    # List should show a warning about the missing METADATA file
+    result = script.pip("list", expect_stderr=True)
+    assert "WARNING: Skipping" in result.stderr
+    assert "due to invalid dist-info directory: missing `METADATA` file" in result.stderr