feat(sessions): add stop run control for cloud runs

[tatoalo]

Problem

There is no way to stop a cloud run from PostHog Code. Esc only interrupts the current response (a turn-level cancel command), so the sandbox keeps running until the backend's inactivity timeout.

stop_cloud_run.mp4

closes #2634

Changes

• add a Stop run button for active cloud sessions next to Continue locally, with a confirmation dialog and a disabled Stopping... state until the run reaches a terminal status.

• add CloudTaskService.stop, exposed through the shared host router, to call the new cancel endpoint and automatically retry transient 503/5xx failures.

• add sessionService.stopCloudRun to handle cloud run cancellation, ignore non-cloud or terminal sessions, and clear the pending state if the request fails.

• add a dedicated TASK_RUN_STOPPED analytics event, separate from the existing TASK_RUN_CANCELLED event used for turn interrupts.

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit a2870c0.}

[stamphog]

This is a 23-file, 782-line behavioral feature (stop cloud run) touching session state, archive flow, API calls, and UI — it exceeded the automated size gate and has zero reviews. The implementation looks reasonable and well-tested, but a change of this scope affecting cloud task lifecycle needs at least one human reviewer before auto-approval.

[greptile-apps]

Security Review

The new stop endpoint can forward the authenticated bearer token to a renderer-supplied apiHost. The fetch target should be derived or validated at the trusted service boundary.

_{Reviews (1): Last reviewed commit: "feat(sessions): add stop run control for..." | Re-trigger Greptile}