feat(firebase_ai): add thinking level to ThinkingConfig

[cynthiajoan]

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[cynthiajoan]

/gemini review

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Code Review

This pull request introduces a thinkingLevel to the ThinkingConfig, allowing users to specify a preset for the model's thinking process. The implementation includes a new ThinkingLevel enum, deprecates the old ThinkingConfig constructor in favor of new factory methods, and updates example usage.

I've found a critical issue in the serialization of thinkingLevel that would cause API requests to fail, along with an incorrect test case that fails to catch this bug. I've also included a couple of minor suggestions to improve documentation and code style. Once the critical issue is addressed, this will be a great addition.