fix(gemini): tolerate missing usage metadata on final response (#4724)

Co-authored-by: Charan Rathore <charan-rathore@users.noreply.github.com>

strands-py/src/strands/models/gemini.py

@@ -677,7 +677,7 @@ class GeminiModel(Model):
                     "data": "TOOL_USE" if tool_used else (candidate.finish_reason if candidate else "STOP"),
                 }
             )
-            if event:
+            if event and event.usage_metadata is not None:
                 yield self._format_chunk({"chunk_type": "metadata", "data": event.usage_metadata})
 
         except genai.errors.ClientError as error:

strands-py/tests/strands/models/test_gemini.py

@@ -1753,3 +1753,48 @@ class TestCountTokens:
         gemini_client.aio.models.count_tokens.assert_not_called()
         assert isinstance(result, int)
         assert result >= 0
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("finish_reason,stop_reason", [("STOP", "end_turn"), ("SAFETY", "guardrail_intervened")])
+async def test_stream_response_without_usage_metadata(
+    gemini_client, model, messages, agenerator, alist, finish_reason, stop_reason
+):
+    response = genai.types.GenerateContentResponse.model_validate(
+        {
+            "candidates": [
+                {
+                    "content": {"role": "model", "parts": [{"text": "Hello"}]},
+                    "finishReason": finish_reason,
+                }
+            ]
+        }
+    )
+    assert response.usage_metadata is None
+    gemini_client.aio.models.generate_content_stream.return_value = agenerator([response])
+
+    chunks = await alist(model.stream(messages))
+
+    assert {"contentBlockDelta": {"delta": {"text": "Hello"}}} in chunks
+    assert chunks[-1] == {"messageStop": {"stopReason": stop_reason}}
+    assert not any("metadata" in chunk for chunk in chunks)
+
+
+@pytest.mark.asyncio
+async def test_stream_response_empty_usage_metadata_is_preserved(gemini_client, model, messages, agenerator, alist):
+    response = genai.types.GenerateContentResponse.model_validate(
+        {
+            "candidates": [{"content": {"role": "model", "parts": [{"text": "Hello"}]}, "finishReason": "STOP"}],
+            "usageMetadata": {},
+        }
+    )
+    gemini_client.aio.models.generate_content_stream.return_value = agenerator([response])
+
+    chunks = await alist(model.stream(messages))
+
+    assert chunks[-1] == {
+        "metadata": {
+            "usage": {"inputTokens": 0, "outputTokens": 0, "totalTokens": 0},
+            "metrics": {"latencyMs": 0},
+        }
+    }