fix: Add validation and documentation for response modalities

[paulelliotco]

This PR addresses issue #386 by adding input validation and clear documentation for response modalities in the live_api_starter.py file.
The changes include:

• Added validate_config() function to check for invalid or unsupported response modalities.
• Improved error handling in the run() method to provide clear feedback for invalid configurations.
• Updated documentation comments to clarify the limitations of response modalities.

[markmcd]

I like your idea here - however we don't typically show client-side validation of configuration for a few reasons.

1. the truth is ultimately on the server-side, so client-side validation code can become incorrect over time (this creates a maintenance burden for us here in the cookbook, or broken samples).
2. data validation is often different per-user, so in this example, a user (either a public user in the future, or a preview user now) would see a false validation error.
3. some amount of validation can be done by using objects (i.e. proto derived types) instead of python dicts - e.g. types.LiveConnectConfig(response_modalities=[types.Modality.AUDIO]) (where types is google.genai.types). it doesn't apply to this specific case, but it's a general reason why we don't include validation in sample code.

So while ['AUDIO'] and ['TEXT'] are the only valid values right now, I don't think it's worth the extra validation code.

If you think that this particular tool is somewhere that people will get caught up, perhaps add a comment along the lines of # While Gemini 2.0 Flash is in experimental preview mode, only one of AUDIO or TEXT may be passed here.

Or if you have a better suggestion that would reach more users (e.g in the docs on ai.google.dev somewhere), please share.

I'll close this now but if you want to send a PR to add a comment, feel free.

[paulelliotco]

Thank you Mark for the detailed feedback!

I completely understand the reasoning behind avoiding client-side validation. I'll add a comment to clarify the limitation of response modalities in the live_api_starter.py file and submit a PR with this update shortly.