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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