fix(agent/gemini-cloudcode): seed delta defaults for reasoning-only stream chunks

_make_stream_chunk built delta_kwargs with only `role`, so a reasoning-only
chunk produced a SimpleNamespace without a `.content` attribute. Downstream
consumers that read `delta.content` then raised AttributeError on Gemini 2.5
Flash, where the thinking delta arrives before any content delta.

Seed `content`, `tool_calls`, `reasoning`, and `reasoning_content` as None
up front, matching the pattern already used in gemini_native_adapter.py.
Key-present arguments still override the defaults.

Fixes #24974
References: Related open PR #24984 (luyao618) applies the same 1-line fix; this PR adds a regression test that #24984 omits
Co-Authored-By: Claude <noreply@anthropic.com>

agent/gemini_cloudcode_adapter.py

@@ -450,7 +450,13 @@ def _make_stream_chunk(
     finish_reason: Optional[str] = None,
     reasoning: str = "",
 ) -> _GeminiStreamChunk:
-    delta_kwargs: Dict[str, Any] = {"role": "assistant"}
+    delta_kwargs: Dict[str, Any] = {
+        "role": "assistant",
+        "content": None,
+        "tool_calls": None,
+        "reasoning": None,
+        "reasoning_content": None,
+    }
     if content:
         delta_kwargs["content"] = content
     if tool_call_delta is not None: