fix: add extra_content property to ToolCall for Gemini thought_signature (#14488)

Commit 43de1ca8 removed the _nr_to_assistant_message shim in favor of
duck-typed properties on the ToolCall dataclass. However, the
extra_content property (which carries the Gemini thought_signature) was
omitted from the ToolCall definition. This caused _build_assistant_message
to silently drop the signature via getattr(tc, 'extra_content', None)
returning None, leading to HTTP 400 errors on subsequent turns for all
Gemini 3 thinking models.

Add the extra_content property to ToolCall (matching the existing
call_id and response_item_id pattern) so the thought_signature round-trips
correctly through the transport → agent loop → API replay path.

Credit to @celttechie for identifying the root cause and providing the fix.

Closes #14488

tests/agent/transports/test_types.py

@@ -200,6 +200,35 @@ class TestToolCallBackwardCompat:
         tc_no_pd = ToolCall(id="1", name="fn", arguments="{}")
         assert getattr(tc_no_pd, "call_id", None) is None
 
+    def test_extra_content_from_provider_data(self):
+        """Gemini thought_signature stored in provider_data is exposed via property."""
+        ec = {"google": {"thought_signature": "SIG_ABC123"}}
+        tc = ToolCall(id="1", name="fn", arguments="{}", provider_data={"extra_content": ec})
+        assert tc.extra_content == ec
+
+    def test_extra_content_none_when_no_provider_data(self):
+        tc = ToolCall(id="1", name="fn", arguments="{}", provider_data=None)
+        assert tc.extra_content is None
+
+    def test_extra_content_none_when_key_absent(self):
+        tc = ToolCall(id="1", name="fn", arguments="{}", provider_data={"call_id": "c1"})
+        assert tc.extra_content is None
+
+    def test_extra_content_getattr_pattern(self):
+        """_build_assistant_message uses getattr(tc, 'extra_content', None).
+
+        This is the exact pattern that was broken before the extra_content
+        property was added — ToolCall lacked the property so getattr always
+        returned None, silently dropping the Gemini thought_signature and
+        causing HTTP 400 on subsequent turns (issue #14488).
+        """
+        ec = {"google": {"thought_signature": "SIG_ABC123"}}
+        tc = ToolCall(id="1", name="fn", arguments="{}", provider_data={"extra_content": ec})
+        assert getattr(tc, "extra_content", None) == ec
+
+        tc_no_extra = ToolCall(id="1", name="fn", arguments="{}")
+        assert getattr(tc_no_extra, "extra_content", None) is None
+
 
 class TestNormalizedResponseBackwardCompat:
     """Test properties that replaced _nr_to_assistant_message() shim."""