[Firebase AI] Add simplified object generation

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

Add logic to allow easier object generation with GenerateObjectAsync on GenerativeModel. Add the IFirebaseDeserializable interface, to allow users to define their own method of deserialization if the reflection logic won't work for their needs.

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Testing

Describe how you've tested these changes.

Running test locally. Proper automated tests will be included in a future PR, as part of another feature.

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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[gemini-code-assist]

Code Review

This pull request introduces GenerateObjectAsync to the Firebase AI SDK, enabling simplified object generation from model responses. It includes a new GenerateObjectResponse<T> struct and a SerializationHelpers utility class to handle JSON deserialization via reflection or the IFirebaseDeserializable interface. The review identified several critical issues: the GenerateObjectResponse struct is vulnerable to NullReferenceException if initialized via the default constructor, the deserialization logic fails when casting null results to value types, Enum.Parse lacks robustness for numeric JSON values, the list deserialization logic does not support generic collection interfaces, and the logging in the serialization helper is overly verbose for production.