Add Gemini Nano support for prompt validation with fallback mechanism

[takahirom]

Thanks for this awesome reference app! I'm going to talk about this app in DevFest Tokyo.

While exploring the codebase, I noticed that Androidify uses Gemini Nano for image validation (validateImageHasEnoughInformation) with a fallback to Firebase AI, but prompt validation
(validatePromptHasEnoughInformation) only uses Firebase AI directly.

I'm curious about the Gemini Nano strategy here. Is there a specific reason why prompt validation doesn't leverage Gemini Nano? If not, this PR adds Gemini Nano support for prompt validation to maintain
consistency with the existing image validation pattern.

Changes

• Added promptTextVerifyNano() to RemoteConfigDataSource with validation prompt
• Added validatePromptHasEnoughInformation() to GeminiNanoGenerationDataSource
• Updated ImageGenerationRepository to try Gemini Nano first, then fallback to Firebase AI
• Updated test mocks

Let me know if there's a specific reason this pattern wasn't used for prompt validation!

[gemini-code-assist]

Summary of Changes

Hello @takahirom, 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 integrates Gemini Nano into the prompt validation workflow, mirroring the existing image validation strategy. Previously, prompt validation exclusively used Firebase AI, while image validation leveraged Gemini Nano with a Firebase AI fallback. This change introduces Gemini Nano as the primary validation method for prompts, with Firebase AI serving as a robust backup, ensuring consistency across validation processes and potentially enhancing performance or capabilities.

Highlights

• Gemini Nano Integration for Prompt Validation: Introduced Gemini Nano support for validating text prompts, aligning with the existing image validation pattern.
• Fallback Mechanism: Implemented a fallback to Firebase AI for prompt validation if Gemini Nano is not successful or enabled.
• New Remote Config Key: Added "promptTextVerifyNano" to RemoteConfigDataSource to fetch the specific prompt for Gemini Nano text validation.
• Updated Repository Logic: Modified ImageGenerationRepository to prioritize Gemini Nano for prompt validation before falling back to Firebase AI.

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[gemini-code-assist]

Code Review

This pull request is a great addition, bringing consistency to the app by adding Gemini Nano support for prompt validation with a fallback mechanism, similar to the existing image validation. The changes are well-structured, including updates to data sources, remote config, and tests.

I've left a couple of suggestions. One is a high-severity recommendation to improve the fallback logic to be more robust, ensuring it triggers not just when Nano is unavailable, but also when Nano's validation fails. This aligns better with the likely intent and improves reliability. The other is a minor suggestion for code conciseness.

Overall, excellent work on improving the app's on-device capabilities!

[takahirom]

Just in case, I've verified that it works.

valid case	invalid case