Optimized the `stop()` method in `CopilotClient` to destroy all active
sessions in parallel using `Promise.all`.

Previously, sessions were destroyed sequentially, leading to a linear
increase in shutdown time as the number of sessions grew. With this
change, the total time for session destruction is approximately the
duration of the slowest individual destruction.

Measurements:
- 10 sessions with 100ms destruction delay:
  - Before: ~1000ms
  - After: ~100ms
- Performance gain: ~10x for 10 sessions.

The individual retry logic and exponential backoff for each session
are preserved. All failures are still collected and reported.

Co-authored-by: AkCodes23 <135016848+AkCodes23@users.noreply.github.com>

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⚡ Bolt: Parallelize session destruction in Node.js SDK