chore(ai): Add integration tests for Live API

[daymxn]

This PR adds Integration tests for the Live API.

The tests currently test the following:

• Text - Text
• Text - Audio
• Audio - Audio
• Audio - Text
• Function Calling
• Function Cancellation
• Interruption
• Incremental (just that it works)

Test are written via the swift testing framework (matching our other [newer] integration tests), and are parametrized to run across both backends and different services. I've removed the global variants of the parameters though, as the vertex model only supports us-central1 anyhow.

#no-changelog

[andrewheard]

I'd recommend splitting off the unit tests into a separate PR. They LGTM.

[daymxn]

I'd recommend splitting off the unit tests into a separate PR. They LGTM.

@andrewheard Oh I already intended to do this (see the PR description for reference). I just have everything here for my own centralization purposes, until I split them off.

[ncooke3]

LGTM on green CI

[andrewheard]

Thanks, @daymxn! You might want to share this with the other SDK teams after we run them for a few days on CI (to verify no flakiness). They could likely re-use your system instructions, audio file and the scenarios for their own integration tests.