fix(socket-mode): await client disconnect on shutdown

[Grmiade]

Problem

SocketModeReceiver.stop() calls SocketModeClient.disconnect() but resolves immediately instead of awaiting it. Applications can therefore log that shutdown completed while the Socket Mode WebSocket close handshake is still in progress.

Solution

Await the Socket Mode client's disconnect promise before resolving SocketModeReceiver.stop(). This makes await app.stop() reflect the actual receiver lifecycle and adds a regression test for the pending disconnect case.

Validation

• npx @biomejs/biome check src/receivers/SocketModeReceiver.ts test/unit/receivers/SocketModeReceiver.spec.ts
• Full build currently blocked by existing strict TypeScript errors in src/Assistant.ts and src/helpers.ts on the cloned main branch.
• Focused Mocha execution is blocked by the existing rewiremock bootstrap error on Node 24.

[changeset-bot]

🦋 Changeset detected

Latest commit: 765ac9e

The changes in this PR will be included in the next version bump.

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR