rtc: match publisher m-sections by mid, not by position (#5122)

* rtc: match publisher m-sections by mid, not by position

A client that stops a transceiver lets the next one recycle its m-line
(JSEP 5.2.2), so a new m-section can come before negotiated ones.
GetUnmatchMediaForOffer stopped at the last answered mid and missed it,
so its codec preferences were never applied.

Bump the protocol so clients know they can stop transceivers.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Update protocol_version.go

* Remove comments from GetUnmatchMediaForOffer function

Removed comments explaining the GetUnmatchMediaForOffer function.

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

pkg/rtc/transportmanager.go

@@ -538,7 +538,7 @@ func (t *TransportManager) createDataChannelsForSubscriber(pendingDataChannels [
 }
 
 func (t *TransportManager) GetUnmatchMediaForOffer(parsedOffer *sdp.SessionDescription, mediaType string) (unmatched []*sdp.MediaDescription, err error) {
-	var lastMatchedMid string
+	matchedMids := make(map[string]struct{})
 	if lastAnswer := t.publisher.CurrentLocalDescription(); lastAnswer != nil {
 		parsedAnswer, err1 := lastAnswer.Unmarshal()
 		if err1 != nil {
@@ -547,22 +547,20 @@ func (t *TransportManager) GetUnmatchMediaForOffer(parsedOffer *sdp.SessionDescr
 			return unmatched, err1
 		}
 
-		for i := len(parsedAnswer.MediaDescriptions) - 1; i >= 0; i-- {
-			media := parsedAnswer.MediaDescriptions[i]
-			if media.MediaName.Media == mediaType {
-				lastMatchedMid, _ = media.Attribute(sdp.AttrKeyMID)
-				break
+		for _, media := range parsedAnswer.MediaDescriptions {
+			if mid, ok := media.Attribute(sdp.AttrKeyMID); ok {
+				matchedMids[mid] = struct{}{}
 			}
 		}
 	}
 
 	for i := len(parsedOffer.MediaDescriptions) - 1; i >= 0; i-- {
 		media := parsedOffer.MediaDescriptions[i]
-		if media.MediaName.Media == mediaType {
-			mid, _ := media.Attribute(sdp.AttrKeyMID)
-			if mid == lastMatchedMid {
-				break
-			}
+		if media.MediaName.Media != mediaType {
+			continue
+		}
+		mid, _ := media.Attribute(sdp.AttrKeyMID)
+		if _, ok := matchedMids[mid]; !ok {
 			unmatched = append(unmatched, media)
 		}
 	}

pkg/rtc/types/protocol_version.go

@@ -16,7 +16,7 @@ package types
 
 type ProtocolVersion int
 
-const CurrentProtocol = 18
+const CurrentProtocol = 19
 
 func (v ProtocolVersion) SupportsPackedStreamId() bool {
 	return v > 0