Abort JoinHandle when futures are cancelled/dropped

[lukipuki]

Summary
This PR implements automatic task abortion for background tasks to ensure they are cleaned up when their handles are dropped. It refactors the management of read, write, processing, heartbeat, and bridge handlers to use tokio_util::task::AbortOnDropHandle.

This prevents resource leaks and ensures clean shutdowns when operations (such as connect()) are cancelled, for example by a tokio::time::timeout.

Related Issues
Fixes #88

Proposed Changes

• Robust Task Management: Refactor ConnectedStreamApi and StreamHandle to use AbortOnDropHandle. This ensures that background tasks are aborted immediately when the API object is dropped or when a connect() future is cancelled.
• Bridge Support: Properly manages the bridge_handle for low-level transports (like BLE), ensuring it is aborted and awaited during disconnection.
• Dependency Update: Enabled the rt feature for tokio-util in Cargo.toml.

Checklist

• Tests pass locally
• Documentation updated if needed