[AT-216] - track the submit source on assistant_submit

[dmitrycnstrc]

A click on a suggested question is an intent submission, but the beacon carried no way to tell it apart from a typed query, so engagement reports either ignored those clicks or counted them as typed input.

sendMessage already knew where an intent came from and passed it to the onAssistantSubmit callback; that value now also travels on the assistant_submit beacon as source (input / suggestion, matching the values the behavioral-actions API documents).

Also documents the tracking surface in the Integration Guide — events, their callbacks, the callbacks / section provider props, and how to send these events from a custom web or native mobile integration, where the click-driven submit is the commonly missed case.

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