[ qingfeng/2020117 ] fix: claim Cashu tokens on customer disconnect (peer-leave handler)

Provider was not claiming earned tokens when customer disconnected
abruptly because there was no peer-leave handler. Now the agent
listens for peer-leave events and immediately runs endSession()
which claims all accumulated tokens. Also wrapped session_end send
in try-catch since the socket may already be closed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
https://github.com/qingfeng/2020117/commit/a8f0d7ec78b496df943d6371cd981b206222e6b4