Basic chat updates surfaces rather than creating new ones

[jacobsimionato]

Basic chat currently has a propensity to use one surface and keep updating it rather than adding new ones. This is now hard to prevent given that FirebaseAiContentGenerator seems to ignore GenUiConfiguration allowUpdate false - #463.

Let's fix these two issues together and ensure that the basic chat behaves as expected.

Example:

Here, I kept completing a form and the agent kept sending text responses and updating the same surface up high in the chat - the one that says "Steven and Ming are invited to Sky Party"

[jacobsimionato]

We should prioritise this because it is also affecting our customers - see #463