Fix apply cancellation and make slash commands cancelable

[RomneyDa]

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#5694 was intended to make apply cancelable but only worked for jetbrains because of how it was set up. This PR fixes for VS Code It also passes abort controllers to slash commands to make them cancelable as well.

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[chezsmithy]

@RomneyDa thank you for all your amazing work on solving for being able to cancel various pieces of the experience. I closed my cancel button fix. Noting that my only remaining issue appears to be cancelling "thinking/reasoning" and ending up with unmatched messages in the history.

[recurseml[bot]]

⚠️ Only 5 files will be analyzed due to processing limits.

[recurseml[bot]]

😱 Found 2 issues. Time to roll up your sleeves! 😱