[AI] AGENTS.md update

[paulb777]

Update AGENTS.md file for #15872 by the following two prompts:

• Update the AGENTS.md files in the FirebaseAI/Sources directory tree based on the changes introduced in ah/ai-generative-model-session branch
• There should be a new AGENTS.md added for the new GenerationSchema+Gemini.swift file

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[paulb777]

/gemini review

[gemini-code-assist]

Code Review

This pull request updates the AGENTS.md documentation to include information about the new GenerativeModelSession and an internal GenerationSchema extension. The updates are generally accurate and helpful. I've provided one suggestion to refine the description of GenerativeModelSession to more accurately reflect its stateless nature, which should improve clarity for developers.

[paulb777]

/gemini review

[gemini-code-assist]

Code Review

The pull request updates the AGENTS.md file in FirebaseAI/Sources to include GenerativeModelSession.swift and adds a new AGENTS.md file in FirebaseAI/Sources/Extensions/Internal for GenerationSchema+Gemini.swift. The changes accurately reflect the new files and their descriptions, improving the documentation of the project structure.