Fix Messenger::stop() to reject pending promises

[matt2005]

Fix Messenger::stop() to reject pending promises and prevent post-shutdown callbacks

Problem:
When AndroidAutoEntity::stop() called messenger_->stop(), only the message queue was cleared but pending receive/send promises remained active. This caused a flood of channel error callbacks (AASDK Error: 30) when the transport closed, leading to re-entrant teardown attempts and potential application freezes.

Solution:

• Messenger::stop() now rejects all pending receive promises with OPERATION_ABORTED
• Messenger::stop() now rejects all pending send promises with OPERATION_ABORTED
• Both operations are dispatched on their respective strands to ensure thread safety

Impact:

• Services will now receive OPERATION_ABORTED errors during shutdown (expected)
• Prevents spurious channel errors after messenger is stopped
• Enables clean, deterministic shutdown without callback floods
• Applications should handle OPERATION_ABORTED as a normal shutdown signal

This is the library-level fix for clean async operation cancellation during teardown. Application-level services should treat OPERATION_ABORTED as debug-level logging.