fix(util): tolerate deleted temporary environment variables

When a `patch_env()` body deletes a variable that was initially absent,
cleanup raises `KeyError` and can mask an exception from the body.
Use `os.environ.pop(name, None)` to restore absence idempotently while
preserving restoration of existing values.

Add 18 regression cases for absent, empty and populated original values,
keep/replace/delete body mutations and normal/exceptional exits.
The focused CLI-backend tests changed from 2 failed and 16 passed to
18 passed. Ruff lint/format and `git diff --check` passed. The full suite
was not run because its required `GixPython` Windows wheel was unavailable.

Agent-Model: unknown

Author: Abedyn

git/util.py

@@ -175,7 +175,7 @@ def patch_env(name: str, value: str) -> Generator[None, None, None]:
         yield
     finally:
         if old_value is None:
-            del os.environ[name]
+            os.environ.pop(name, None)
         else:
             os.environ[name] = old_value
 

test/test_patch_env.py

@@ -0,0 +1,39 @@
+"""Regression tests for temporary environment variable changes."""
+
+import os
+
+import pytest
+
+from git.util import patch_env
+
+
+@pytest.mark.parametrize("original", [None, "", "original"])
+@pytest.mark.parametrize("change", ["keep", "replace", "delete"])
+@pytest.mark.parametrize("raise_error", [False, True])
+def test_patch_env_restores_original_state(monkeypatch, original, change, raise_error):
+    name = "GITPYTHON_TEST_PATCH_ENV"
+    if original is None:
+        monkeypatch.delenv(name, raising=False)
+    else:
+        monkeypatch.setenv(name, original)
+
+    def change_environment():
+        with patch_env(name, "temporary"):
+            assert os.environ[name] == "temporary"
+            if change == "replace":
+                os.environ[name] = "changed"
+            elif change == "delete":
+                del os.environ[name]
+            if raise_error:
+                raise RuntimeError("context body failed")
+
+    if raise_error:
+        with pytest.raises(RuntimeError, match="context body failed"):
+            change_environment()
+    else:
+        change_environment()
+
+    if original is None:
+        assert name not in os.environ
+    else:
+        assert os.environ[name] == original