fix(agent): extract usageMetadata from Gemini streaming finish chunks

Gemini streaming sessions reported 0 tokens because
translate_stream_event did not extract usageMetadata from the finish
event.  Mirrors the non-streaming translate_gemini_response path.

Supersedes #15264.

tests/agent/test_gemini_streaming_usage.py

@@ -0,0 +1,77 @@
+"""Tests for Gemini streaming usageMetadata extraction.
+
+The streaming path must extract token counts from usageMetadata on
+the finish chunk, matching the non-streaming translate_gemini_response
+behavior.
+"""
+import unittest
+
+from agent.gemini_native_adapter import translate_stream_event
+
+
+class TestGeminiStreamingUsageMetadata(unittest.TestCase):
+    """Verify usageMetadata is attached to streaming finish chunks."""
+
+    def _finish_event(self, usage_meta=None):
+        """Create a Gemini streaming event with finishReason and optional usageMetadata."""
+        event = {
+            "candidates": [{
+                "content": {"parts": [], "role": "model"},
+                "finishReason": "STOP",
+            }],
+        }
+        if usage_meta is not None:
+            event["usageMetadata"] = usage_meta
+        return event
+
+    def _get_usage(self, chunks):
+        """Extract usage from a list of chunks, if present."""
+        for c in chunks:
+            usage = getattr(c, "usage", None)
+            if usage is not None:
+                return usage
+        return None
+
+    def test_usage_attached_on_finish(self):
+        """Finish chunk should carry usage from usageMetadata."""
+        event = self._finish_event({
+            "promptTokenCount": 100,
+            "candidatesTokenCount": 50,
+            "totalTokenCount": 150,
+            "cachedContentTokenCount": 20,
+        })
+        chunks = translate_stream_event(event, model="gemini-2.5-pro", tool_call_indices={})
+        usage = self._get_usage(chunks)
+        self.assertIsNotNone(usage, "Finish chunk should have usage")
+        self.assertEqual(usage.prompt_tokens, 100)
+        self.assertEqual(usage.completion_tokens, 50)
+        self.assertEqual(usage.total_tokens, 150)
+        self.assertEqual(usage.prompt_tokens_details.cached_tokens, 20)
+
+    def test_no_usage_when_metadata_absent(self):
+        """When usageMetadata is missing, finish chunk should not have usage."""
+        chunks = translate_stream_event(
+            self._finish_event(), model="gemini-2.5-pro", tool_call_indices={}
+        )
+        self.assertIsNone(self._get_usage(chunks))
+
+    def test_partial_metadata_defaults_to_zero(self):
+        """Missing fields in usageMetadata should default to 0."""
+        event = self._finish_event({
+            "promptTokenCount": 200,
+            "totalTokenCount": 200,
+            # candidatesTokenCount and cachedContentTokenCount missing
+        })
+        chunks = translate_stream_event(event, model="gemini-2.5-pro", tool_call_indices={})
+        usage = self._get_usage(chunks)
+        self.assertIsNotNone(usage)
+        self.assertEqual(usage.prompt_tokens, 200)
+        self.assertEqual(usage.completion_tokens, 0)
+        self.assertEqual(usage.prompt_tokens_details.cached_tokens, 0)
+
+    def test_empty_metadata_no_usage(self):
+        """Empty usageMetadata dict should not create usage."""
+        chunks = translate_stream_event(
+            self._finish_event({}), model="gemini-2.5-pro", tool_call_indices={}
+        )
+        self.assertIsNone(self._get_usage(chunks))