Handle role conflict 487 error responses

[bgardner8008]

Implements the missing half of RFC 8445 §7.2.5.1: handle 487 Role Conflict error responses.

The existing code (commit 2c04474) handles role conflicts detected in inbound Binding requests (§7.3.1.1), but did not handle 487 error responses received when we are the requester.

This change adds handling for the requester side:

• Verify message integrity of the 487 response
• Switch ICE role (controlling <-> controlled)
• Regenerate the tiebreaker value (required by RFC 8445 §7.2.5.1)
• Re-enqueue the candidate pair in the triggered-check queue with state Waiting

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.07%. Comparing base (c5b305b) to head (4cb7c19).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #892      +/-   ##
==========================================
- Coverage   88.57%   88.07%   -0.51%     
==========================================
  Files          44       44              
  Lines        5540     5625      +85     
==========================================
+ Hits         4907     4954      +47     
- Misses        439      463      +24     
- Partials      194      208      +14     

Flag	Coverage Δ
go	88.07% <100.00%> (-0.51%)	⬇️

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