fix(ai): add documentation around the number of tool call allowed

[Lyokone]

Description

Add client-side validation that throws an ArgumentError when Tool.functionDeclarations() is mixed with other tool types (googleSearch, codeExecution, urlContext) in the same request. The Gemini API does not support this combination and previously returned a cryptic server error. Also adds doc comments on the Tool factory constructors documenting this limitation.

Related Issues

• Fixes [firebase_ai]: Grounding with Google not working in Flutter #17687

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change.

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