feat(gemini): add thought signature preservation for thinking models

[pgrayy]

Description

Related Issues

Documentation PR

Type of Change

Bug fix
New feature
Breaking change
Documentation update
Other (please describe):

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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[codecov]

Codecov Report

❌ Patch coverage is 72.72727% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/strands/event_loop/streaming.py	0.00%	2 Missing and 2 partials ⚠️
src/strands/models/gemini.py	86.66%	1 Missing and 1 partial ⚠️

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