fix: route publisher negotiation errors to reconnect instead of leaking them

[RaphaelFakhri]

Summary

Fixes #1093.

Transport.negotiate is a debounced function. The debouncer discards the future that createAndSendOffer() returns, so a rejection (for example setLocalDescription failing with "The order of m-lines in subsequent offer doesn't match") became an unhandled error. The try/catch in Engine.negotiate never saw it, because publisher.negotiate(null) is not awaited and returns immediately. As a result, the reconnect that the catch block was meant to trigger never ran, and apps that install PlatformDispatcher.onError saw the raw NegotiationError.

This change:

• Adds Transport.onNegotiationError, and the debounced callback catches errors from createAndSendOffer() and reports them through it.
• Sets the callback in Engine to the previous catch-block logic: set fullReconnectOnNext for a NegotiationError, then call handleReconnect(negotiationFailed).
• Removes the dead try/catch from Engine.negotiate.

Test plan

• New test/core/transport_negotiate_test.dart:
  • a peer connection whose setLocalDescription rejects reports a NegotiationError to onNegotiationError and leaks no uncaught zone error;
  • a normal negotiation sends one offer and reports nothing.
• Before the change the first test fails with an uncaught NegotiationError. After the change it passes.
• flutter analyze, dart format . --set-exit-if-changed, import_sorter and the full flutter test suite (440 tests) pass.
• Added a .changes entry.

[CLAassistant]

All committers have signed the CLA.

[RaphaelFakhri]

Valid finding, fixed in the new commit. setRemoteDescription now sends the offer deferred by renegotiate through the same error-reporting path as the debounced negotiate, so a rejected deferred offer reaches onNegotiationError and triggers the reconnect. Direct createAndSendOffer calls, such as the one in resumeConnection, still propagate their errors to the caller. Two tests cover this: one starts with a local offer, sets renegotiate, and rejects the post-answer offer; the other checks that a direct call still throws.