config(chat): try opt in implicit @workspace context once if users are in treatment group

[Will-ShaoHua]

Problem

Most users might have disabled workspace context due to previous sub-process performance issue, therefore #6098 experiment has too few datapoints.

Solution

As we've been actively working on fixes to improve the amazon q helper sub-process performance, the team decided to try turn on (only once) workspace context for a small fraction of all users (20%) to collect few more datapoints.

Screen.Recording.2025-01-27.at.12.49.36.PM.mov

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[Will-ShaoHua]

jetbrains pr aws/aws-toolkit-jetbrains#5302