fix: key _simulcast_track_map by SDP MID availability, not RTP MID extension

Use mid.has_value() (SDP a=mid) as the sole condition for building the
"mid:rid" composite key in _simulcast_track_map, instead of requiring
both has_mid_extension (RTP MID header extension) and mid.has_value().

Previously, if the SDP contained a=mid but the RTP MID extension was not
negotiated, the map key was stored as "rid" while MID+RID format headers
(mid:<mid>:rid:<rid>:...) expected "mid:rid", causing a lookup mismatch.

Update comments in webrtc_stream.h and webrtc_stream.cpp to reflect that
the key condition is based on the SDP a=mid identifier, not the RTP MID
header extension.

Author: SangwonOh

src/projects/providers/webrtc/webrtc_stream.cpp

@@ -271,7 +271,7 @@ namespace pvd
 
 			if (has_rid_extension)
 			{
-				// Key: "mid:rid" when MID extension is present to disambiguate across m= sections
+				// Key: "mid:rid" when the SDP a=mid identifier is available to disambiguate across m= sections
 				// that reuse the same RID values; otherwise just "rid".
 				// RID values are [a-zA-Z0-9\-_]+ and MID tokens contain no ':', so ':' is a safe separator.
 				//
@@ -280,7 +280,7 @@ namespace pvd
 				//   MID="video0", RID="low"  → key = "video0:low"
 				//   no MID,       RID="high" → key = "high"
 				auto mid = answer_media_desc->GetMid();
-				auto key = (has_mid_extension && mid.has_value())
+				auto key = mid.has_value()
 							? (mid.value() + ":" + rid_attr->GetId())
 							: rid_attr->GetId();
 				_simulcast_track_map[key] = track->GetId();

src/projects/providers/webrtc/webrtc_stream.h

@@ -132,8 +132,8 @@ namespace pvd
 		std::map<uint32_t, bool> _sent_sequence_header;
 
 		// RID to track ID mapping
-		// Key: "mid:rid" when MID extension is present (disambiguates across m= sections with reused RIDs),
-		// or just "rid" when MID is unavailable. Each key is unique, so a single track ID per entry suffices.
+		// Key: "mid:rid" when the SDP a=mid identifier is available (disambiguates across m= sections with reused RIDs),
+		// or just "rid" when no SDP MID is available. Each key is unique, so a single track ID per entry suffices.
 		std::map<ov::String, uint32_t> _simulcast_track_map;
 
 		ov::String _oven_capabilities;