fix(firebaseai): Add thoughtSignature during chat session function call

[cynthiajoan]

Description

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Related Issues

#17887

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[cynthiajoan]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces support for thoughtSignature and isThought properties within Part objects, which is a key feature for enabling more advanced model interactions. The implementation is well-executed, featuring a nice refactoring of the toJson serialization logic into the base Part class to improve maintainability and reduce code duplication. The tests have also been updated effectively to cover the new functionality and even improve upon existing test quality. I have identified two minor opportunities for improvement in the JSON parsing logic to make it more robust against potential runtime errors, and have provided suggestions for these.