Fix null request ID when unsubscribing from streams

[anshitraj]

Summary

• assign a unique request ID to stream UNSUBSCRIBE messages
• add a regression test covering the acknowledgement path and pending-request cleanup

Problem

StreamConnectionWrapper.unsubscribe currently sends the request without an ID. Binance echoes that as "id": null, and onWebSocketText then calls getAsString() on JsonNull, raising UnsupportedOperationException.

Using a generated ID matches the other control messages in this class and lets the acknowledgement be correlated and removed from pendingRequest normally.

Fixes #247.

Testing

A focused unit test verifies that the unsubscribe payload contains a non-null ID, its acknowledgement does not throw, and the pending request is removed. GitHub Actions will run the repository's Maven verification.