test(mcp): pin PROXY_BASE_URL off in MCP OAuth tests

get_request_base_url resolves PROXY_BASE_URL ahead of request.base_url, and
import litellm loads a .env from any parent directory of the checkout, so a
developer with that var set (git worktrees under <repo>/.claude/worktrees/
inherit the main checkout's .env this way) had the mocked request origins
silently ignored and these tests failed on unmodified code.

tests/test_litellm/proxy/_experimental/mcp_server/conftest.py

@@ -20,3 +20,15 @@ def _hermetic_server_root_path():
     finally:
         if saved is not None:
             os.environ["SERVER_ROOT_PATH"] = saved
+
+
+@pytest.fixture(autouse=True)
+def _hermetic_proxy_base_url(monkeypatch):
+    """Isolate MCP discovery tests from a developer's ``PROXY_BASE_URL``.
+
+    ``get_request_base_url`` resolves that env var ahead of the request's own base URL, and
+    ``import litellm`` loads a ``.env`` from any parent directory of the checkout, so a set value
+    silently overrides the origins these tests drive through the request. Clearing it here pins
+    the request-derived origin; a test that exercises the env override sets the value itself.
+    """
+    monkeypatch.delenv("PROXY_BASE_URL", raising=False)

tests/test_litellm/proxy/management_endpoints/test_mcp_management_endpoints.py

@@ -1556,6 +1556,14 @@ class TestTeamScopedMCPServerAccess:
 
 
 class TestTemporaryMCPSessionEndpoints:
+    @pytest.fixture(autouse=True)
+    def unset_proxy_base_url(self, monkeypatch):
+        """``get_request_base_url`` resolves ``PROXY_BASE_URL`` ahead of ``request.base_url``, so a
+        developer with that env var set (a ``.env`` in any parent directory of the checkout is
+        enough, since ``import litellm`` runs dotenv) would otherwise silently bypass the mocked
+        ``request.base_url`` these tests assert on."""
+        monkeypatch.delenv("PROXY_BASE_URL", raising=False)
+
     def test_inherit_credentials_from_existing_server(self):
         payload = NewMCPServerRequest(
             server_id="server-123",