fix(grok): close the token-endpoint response like codex does

The response was left for the garbage collector, which on Python 3.14 surfaces as "Exception ignored while finalizing" urllib3 noise at interpreter shutdown. codex._post_form already context-manages its response.

tests/test_grok_auth.py

@@ -56,6 +56,7 @@ def test_post_form_returns_parsed_body() -> None:
     resp = mock.MagicMock()
     resp.status_code = 200
     resp.content = b'{"access_token": "tok"}'
+    resp.__enter__.return_value = resp
 
     with mock.patch.object(requests, "post", return_value=resp) as post:
         data = grok._post_form({"grant_type": "refresh_token"})
@@ -68,6 +69,7 @@ def test_post_form_raises_on_http_error() -> None:
     resp = mock.MagicMock()
     resp.status_code = 400
     resp.text = "invalid_grant"
+    resp.__enter__.return_value = resp
 
     with (
         mock.patch.object(requests, "post", return_value=resp),