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### Motivation and Context Fixes #13480 When using Gemini models (2.5 Pro / 3 Pro) with thinking enabled, the API returns a `thought_signature` on function call `Part` objects. Per [Google's Thought Signatures documentation](https://ai.google.dev/gemini-api/docs/thought-signatures), this signature **must** be included in subsequent requests. Without it, follow-up turns with tool results fail with: ``` HTTP 400: Function call is missing a thought_signature in functionCall parts ``` The .NET connector was already fixed in #13418. This PR applies the equivalent fix to the Python connector. ### Description **Response parsing** (capture `thought_signature`): - `google_ai_chat_completion.py`: When creating `FunctionCallContent` from response parts, capture `part.thought_signature` into `FunctionCallContent.metadata` (both non-streaming and streaming paths) - `vertex_ai_chat_completion.py`: Same capture via `part_dict.get("thought_signature")` for Vertex AI **Request building** (restore `thought_signature`): - `google_ai/services/utils.py`: In `format_assistant_message()`, when a `FunctionCallContent` has `thought_signature` in its metadata, construct the `Part` directly with the `thought_signature` parameter instead of using `Part.from_function_call()` - `vertex_ai/services/utils.py`: In `format_assistant_message()`, include `thought_signature` in the `Part.from_dict()` dictionary when present **Backward compatible**: When `thought_signature` is not present (thinking disabled or older models), behavior is identical to before. ### Test Coverage Added 4 new tests: - `test_format_assistant_message_with_thought_signature` (Google AI) — verifies round-trip preservation - `test_format_assistant_message_without_thought_signature` (Google AI) — verifies backward compatibility - `test_format_assistant_message_with_thought_signature` (Vertex AI) — verifies round-trip preservation - `test_format_assistant_message_without_thought_signature` (Vertex AI) — verifies backward compatibility All 110 existing Google connector tests continue to pass. ### Contribution Checklist - [x] The code builds clean without any errors or warnings - [x] The PR follows the [SK Contribution Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md) - [x] All unit tests pass, and I have added new tests - [x] I didn't break anyone 😄 --------- Co-authored-by: giulio-leone <[email protected]> Co-authored-by: Copilot <[email protected]>