restore theme dev analytics

[EvilGenius13]

WHY are these changes introduced?

When users press Ctrl+C to exit shopify theme dev, the process immediately calls process.exit(), terminating before the oclif post-run hook can send analytics events. This means we lose telemetry data for theme dev sessions.

The backgroundJobPromise was originally typed as Promise<never> because it was only meant to reject on error (never resolve). This meant there was no mechanism to gracefully complete the promise chain on user-initiated exit.

WHAT is this pull request doing?

Fixes analytics not being sent when exiting theme dev via Ctrl+C by implementing graceful shutdown:

1. Changes the Ctrl+C handler to resolve the background job promise instead of calling process.exit() directly
2. After the main promise chain completes, polls postRunHookHasCompleted() with a 5-second timeout
3. Calls process.exit(0) only after analytics have been sent (or timeout reached)

How to test your changes?

1. Run shopify theme dev against a development store
2. Press Ctrl+C to exit
3. Verify analytics event is sent

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing, so I've added a changelog entry with pnpm changeset add