[FirebaseAI] Change Participant.system case to Participant.model case

[YoungHypo]

According to the Firebase AI Logic document, the role in ModelContent for chat history should be either user or model. Therefore, Participant.system is suggested to be updated to Participant.model to maintain consistency.

[gemini-code-assist]

Summary of Changes

Hello @YoungHypo, 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 aims to improve consistency within the Firebase AI example applications. It standardizes the representation of AI-generated messages by updating the Participant enum to use model instead of system, adhering to the Firebase AI Logic document's guidelines for chat history roles. This is primarily a naming and consistency change across the codebase.

Highlights

• Enum Renaming: The Participant enum in ChatMessage.swift has been updated to rename the system case to model. This change aligns the enum definition with the recommended model role for AI participants in chat history, as specified by the Firebase AI Logic document.
• Consistent Participant Role Usage: All instances across the ChatExample and FunctionCallingExample where Participant.system was previously used to denote AI-generated messages (e.g., in sample data, when creating pending messages, and within UI rendering logic) have been updated to consistently use the new Participant.model case.

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[gemini-code-assist]

Code Review

This pull request updates the Participant enum by renaming the system case to model, aligning with the Firebase AI API. The changes are applied consistently across the example apps. To further improve clarity, I've suggested renaming local variables from systemMessage to modelMessage.