## Summary
This PR fixes GStreamer interoperability issues during WebRTC/WHEP
negotiation with ZLMediaServer.
GStreamer could fail to establish the connection for two separate
reasons:
1. ZLMediaServer generated a non-compliant ICE `ufrag`. The generated
value contained `_`, which is not a valid ICE character, so GStreamer
rejected the SDP.
2. ZLMediaServer did not correctly handle `bundle-only` offers and could
answer an accepted bundled m-line with `port=0`, which caused the later
WHEP negotiation to fail.
## Changes
- Generate ICE `ufrag` values using ICE-compliant characters only.
- Preserve and handle `a=bundle-only` correctly during SDP parsing and
answer generation.
- Return `port=9` instead of `port=0` for accepted bundled m-lines.
- Add regression coverage for `bundle-only` SDP handling.
- URL-encode `delete_webrtc` query parameters in the returned `Location`
header so ICE-safe identifiers remain round-trippable over HTTP.
## Validation
- Built with WebRTC and SCTP enabled.
- Added regression test: `test_webrtc_regression`
- Verified:
- ICE-safe identifier round-trip through `delete_webrtc`
- `bundle-only` SDP answer generation
