test: isolate credentials and create_pr tests from MCP env leakage

Fix 8 suite failures triggered when GITEA_MCP_CONFIG/GITEA_MCP_PROFILE
are set in the operator environment.

- TestGetAuthHeader: clear MCP config env and DYNAMIC_CONFIGS in setUp
- TestCreatePR/TestIssueLocking: add CREATE_PR_ENV author fixture and
  patch reviewer-stop gate so tests reach intended code paths

Closes #227

tests/test_credentials.py

@@ -107,6 +107,24 @@ class TestGetCredentials(unittest.TestCase):
 class TestGetAuthHeader(unittest.TestCase):
     """Test the get_auth_header function."""
 
+    def setUp(self):
+        self._saved_env = os.environ.copy()
+        self._saved_configs = gitea_auth.DYNAMIC_CONFIGS.copy()
+        for key in (
+            "GITEA_MCP_CONFIG",
+            "GITEA_MCP_PROFILE",
+            "GITEA_TOKEN",
+            "GITEA_TOKEN_PRGS",
+        ):
+            os.environ.pop(key, None)
+        gitea_auth.DYNAMIC_CONFIGS.clear()
+
+    def tearDown(self):
+        os.environ.clear()
+        os.environ.update(self._saved_env)
+        gitea_auth.DYNAMIC_CONFIGS.clear()
+        gitea_auth.DYNAMIC_CONFIGS.update(self._saved_configs)
+
     @patch("gitea_auth.get_credentials", return_value=("user", "pass"))
     def test_returns_basic_header(self, _cred):
         header = gitea_auth.get_auth_header("example.com")