Add support for system instructions in the ondevice sdk.

[VinayGuthal]

Adds support for systemInstruction when generating content using on-device and hybrid inference modes.

Summary of Changes

• firebase-ai-ondevice-interop: Added an optional systemInstruction: TextPart? property to GenerateContentRequest and updated api.txt.
• firebase-ai-ondevice: Updated GenerateContentRequest.toMlKit() in Converters.kt to convert the interop TextPart into an ML Kit SystemInstruction.
• firebase-ai:
  • Passed systemInstruction from GenerativeModel.Builder to OnDeviceGenerativeModelProvider.
  • Updated OnDeviceGenerativeModelProvider to validate that systemInstruction contains only text parts (throwing FirebaseAIOnDeviceInvalidRequestException for non-text parts), concatenate multiple TextPart instances with a newline, and attach it to the on-device request.
• Tests & Changelogs: Added unit tests in ConvertersTest, GenerativeModelBuilderTests, and OnDeviceGenerativeModelProviderTests, and updated CHANGELOG.md across all three modules.

Tested updating the quickstart app to add testing functionality for system instruction at firebase/quickstart-android#2846

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[rlazo]

/gemini review

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Code Review

This pull request introduces support for systemInstruction in on-device content generation requests across the firebase-ai-ondevice-interop, firebase-ai-ondevice, and firebase-ai modules. It includes API updates, converters, validation logic to ensure only text parts are used in system instructions, and comprehensive unit tests. The feedback suggests optimizing the validation logic in OnDeviceGenerativeModelProvider to avoid filtering the parts list twice in the happy path.

[rlazo]

Shouldn't the other api.txt files be updated too?

[VinayGuthal]

Shouldn't the other api.txt files be updated too?

I have run the apitext command for all sdks.