[googlechat] Improve OS notification display

Google Chat have some caveats regarding OS notification display when pushing messages.

* Desktop OS notification only display plain-text content (`text` field): https://developers.google.com/workspace/chat/api/reference/rpc/google.chat.v1#google.chat.v1.Message.FIELDS.string.google.chat.v1.Message.text
* Mobile OS notification only display fallback content (`fallback_text_` field): https://developers.google.com/workspace/chat/api/reference/rpc/google.chat.v1#google.chat.v1.Message.FIELDS.string.google.chat.v1.Message.fallback_text

So, it would be nice to support:
* both `text` and `cardV2`: I think having a dedicated `text` field along the current `cardsV2` one. AND ALSO an option to just reuse notification message as currently when no `cardsV2` is provided.
* `fallback_text`: I think having a dedicated `fallback_text` (or any other case) field along the current `cardV2` one. AND ALSO an option to juste reuse notification message.

Start by reviewing the Google Chat message payload fields named in the issue: text, cardsV2, and fallback_text, along with the current notification message behavior when cardsV2 is absent. Define how dedicated text and fallback_text values coexist with the existing reuse option, then verify that desktop and mobile notifications receive the intended content.

Written by the indexing model from the issue text.