fix(core): preserve thought_signature on TextContent in Gemini streaming path (#9461)

get_content() was only setting signature on ReasoningContent items.
When Gemini returns a function call with thought_signature but no
ReasoningContent (e.g. include_thoughts=False), the signature was
stored on self.thinking_signature but never attached to TextContent.
This caused "missing thought_signature in functionCall parts" errors
when the message was echoed back to Gemini on the next turn.

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Co-authored-by: Letta <noreply@letta.com>

letta/interfaces/gemini_streaming_interface.py

@@ -97,9 +97,11 @@ class SimpleGeminiStreamingInterface:
 
     def get_content(self) -> List[ReasoningContent | TextContent | ToolCallContent]:
         """This is (unusually) in chunked format, instead of merged"""
+        has_reasoning = any(isinstance(c, ReasoningContent) for c in self.content_parts)
         for content in self.content_parts:
             if isinstance(content, ReasoningContent):
-                # This assumes there is only one signature per turn
+                content.signature = self.thinking_signature
+            elif isinstance(content, TextContent) and not has_reasoning and self.thinking_signature:
                 content.signature = self.thinking_signature
         return self.content_parts