Leaving a session open and idle (page open, no activity at all) eventually pops an error dialog:

    UncaughtPromiseError > RequestPendingError
    REGISTER request already in progress, waiting for final response
        at Registerer.register (sip.js)
        at Registerer.register (registerer.js)
        at UserAgent.attemptReconnection (user_agent_service.js)

When the WebSocket transport drops while a REGISTER is in flight (which happens on an idle tab: SIP.js sends a periodic re-REGISTER before the registration expires, and the socket may be closed by an idle timeout or by the machine going to sleep in the meantime), the final response never comes back. SIP.js only clears its internal `waiting` flag from the REGISTER response callbacks (onAccept/onReject/onRedirect); it is never reset on transport loss or request timeout. The Registerer is then stuck `waiting` forever, and every subsequent register() rejects with a RequestPendingError.

On top of that, our wrapper's register() did not return the SIP.js promise, and attemptReconnection() called it without awaiting, so the rejection escaped the surrounding try/catch and surfaced as an unhandled promise rejection. Worse, the WebSocket error was resolved right after, so the user appeared reconnected while VoIP registration was actually dead until the page was reloaded.

This commit makes register() recreate the underlying SIP.js Registerer when it is stuck `waiting` (a clean instance starts with waiting=false), and return the promise so callers can await it. attemptReconnection() now awaits it, so any rejection goes through the existing retry/back-off logic instead of bubbling up as an uncaught error.

The recreation is intentionally conditional: disposing a healthy registerer would send an unregister (REGISTER expires=0) racing with the fresh register (expires=600) and could leave us unregistered, so we only recreate when a request is actually stuck.

Forward-Port-Of: odoo/enterprise#120487
Forward-Port-Of: odoo/enterprise#119701