[FirebaseAI] Add support for Server Prompt Templates

[a-maurice]

Description

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

Add logic to support using Server Prompt Templates to generate content. Add automated tests. Also, update imagen tests to use 4 instead of 3.

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Testing

Describe how you've tested these changes.

Running 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]

Summary of Changes

Hello @a-maurice, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the Firebase AI SDK by integrating support for server prompt templates. This feature allows developers to define and manage AI prompts on the server, then invoke them from the client with dynamic inputs. This abstraction simplifies prompt engineering, promotes reusability, and enables more flexible and powerful AI-driven applications, covering both text generation and image creation scenarios.

Highlights

• Server Prompt Template Support: Introduced new functionality to support using server-side prompt templates for both generative AI models (e.g., Gemini) and image generation models (e.g., Imagen) within the Firebase AI SDK.
• New Model Classes: Added TemplateGenerativeModel and TemplateImagenModel classes to interact with these server templates, providing methods for content and image generation.
• FirebaseAI Integration: New public methods GetTemplateGenerativeModel and GetTemplateImagenModel were added to the FirebaseAI class, allowing easy instantiation of the new template-based models.
• API Endpoint Handling: Implemented new helper functions in HttpHelpers.cs to construct appropriate API URLs for server prompt templates, supporting both VertexAI and GoogleAI backends.
• Automated Testing: Comprehensive automated tests were added to validate the functionality of generating content and images using server prompt templates, including streaming capabilities.
• Imagen Model Update: Updated existing Imagen model tests to use the newer imagen-4.0-generate-001 model instead of imagen-3.0-generate-002.

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[gemini-code-assist]

Code Review

This pull request introduces support for Server Prompt Templates for both generative and Imagen models. The changes include new model classes (TemplateGenerativeModel and TemplateImagenModel), corresponding factory methods in FirebaseAI, and helper methods for URL construction. Additionally, automated tests have been added to verify the new functionality. The implementation is solid, but I have a few suggestions to improve code organization and reduce duplication, such as moving a class to its own file and refactoring a helper method for better clarity.