fix(webrtc): resolve remote stream attachment issue and concurrent modification error

[2001arman]

Summary

This PR fixes issues in the WebRTC video call flow where the remote stream was not properly attached, resulting in missing video/audio on the receiver side.

Issues

• Remote stream was null due to incorrect SDP metadata key
• Concurrent modification error when iterating ICE candidates during async processing

Changes

• Updated sdpStreamMetadataKey from:
  • org.matrix.msc3077.sdp_stream_metadata
    → to:
  • sdp_stream_metadata
• Fixed concurrent modification issue by iterating over a copied list of _remoteCandidates

Impact

• Remote media stream now attaches correctly
• Prevents runtime crash: Concurrent modification during iteration
• Improves stability of WebRTC signaling and ICE candidate handling

Notes

• The key change aligns with the expected stable SDP metadata format (post-MSC3077)
• Candidate handling is now safe for async execution

Testing

• Verified video call connects successfully
• Remote video/audio stream is properly rendered
• No concurrent modification errors during ICE candidate processing

[2001arman]

Hi @krille-chan, could you please review my PR? This PR fixes a WebRTC video call bug where the remote stream attachment was null due to an incorrect sdpStreamMetadataKey value. It also includes a minor fix for a concurrent modification issue when processing ICE candidates. Please let me know if there’s anything I might have missed. Thanks!

[krille-chan]

@td-famedly you are probably best to review this?