fix(ai): Add missing available to extension

[daymxn]

It looks like the extension of FirebaseAI was missing an @available quantifier that matched FirebaseAI, and this was causing a compile error. This PR adds that @available marker to the extension.

#no-changelog

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[ncooke3]

LGTM on green

[ncooke3]

Please check that integration tests pass locally. Then merge and run tag update command.