[FirebaseAI] Add AgentPlatform backend

[a-maurice]

Description

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

Add the Agent Platform backend, which replaces the Vertex AI one. Note that the old one is deprecated, and will be removed in the future.

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Testing

Describe how you've tested these changes.

Running tests locally

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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 support for the new AgentPlatform backend provider, deprecating the VertexAI backend configuration in favor of AgentPlatform (which defaults to the 'global' location). The changes update Citation.cs, FirebaseAI.cs, GenerativeModel.cs, and LiveGenerativeModel.cs to handle the new provider. However, a critical issue was identified: HttpHelpers.cs (specifically GetURL and GetTemplateURL) has not been updated to support AgentPlatform, which will cause a runtime NotSupportedException when using this backend.

[a-maurice]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces the Agent Platform backend provider to the Firebase AI SDK, deprecating the existing Vertex AI backend. It updates the SDK core logic, URL generation, citation handling, and the test application to support this new backend. The review feedback points out that the @deprecated tag used in the XML documentation for VertexAI is a Javadoc tag and is invalid in C#. It suggests replacing it with the standard tag to ensure proper rendering in C# IDEs.