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>
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