infra(ai): Add script to run integration tests locally

[daymxn]

This PR adds a script (scripts/tests/ai/run.sh) which can be used locally to run the integration tests for Firebase AI.

The script does the following:

• Decrypts the secret files
• Runs the integration tests
• Deletes the decrypted secret files, regardless of the result of the tests
• Propagates any error from the integration tests back up

You can explicitly provide an Xcode version and a platform target to the script, or let it automatically determine the arguments to use.

The script includes documentation, proper error handling, and debug logging.

#no-changelog