fix(models): Drop stale thought signatures

[its-amann]

Fixes #3693

Summary

• Keeps only the newest thought_signature in outgoing request contents.
• Clears earlier signatures before cache handling or request dispatch, so the payload does not carry values the next request no longer uses.
• Adds a regression test that builds multiple signed parts across the request history and verifies only the newest signature is sent.

Testing

• uv run --extra dev pyink src\google\adk\models\google_llm.py tests\unittests\models\test_google_llm.py
• uv run --extra dev ruff check src\google\adk\models\google_llm.py tests\unittests\models\test_google_llm.py
• uv run python -m pytest tests\unittests\models\test_google_llm.py::test_generate_content_async_keeps_only_latest_thought_signature
• uv run python -m pytest tests\unittests\models\test_google_llm.py::test_generate_content_async_keeps_only_latest_thought_signature tests\unittests\models\test_google_llm.py::test_generate_content_async tests\unittests\models\test_google_llm.py::test_generate_content_async_stream tests\unittests\models\test_google_llm.py::test_generate_content_async_stream_preserves_thinking_and_text_parts

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[adk-bot]

Response from ADK Triaging Agent

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[rohityan]

Hi @its-amann , Thank you for your contribution! We appreciate you taking the time to submit this pull request. Your PR has been received by the team and is currently under review. We will provide feedback as soon as we have an update to share.

[rohityan]

Hi @GWeale , can you please review this.