feat: add diarize field to mediajson Start event

[bgrozev]

Adds an optional diarize: Boolean? field to the media-json Start event so the videobridge can enable speaker diarization per endpoint on the transcriber start event. Defaults to null and is omitted from the wire (NON_NULL inclusion) when unset, so the format is unchanged for existing callers. The opus-transcriber-proxy already reads start.diarize (an explicit boolean overrides its global config).

Part of a multi-repo change enabling per-endpoint speaker diarization. The four Maven repos share the branch diarization-endpoint-control so CI builds them together; no dependency version bumps are included.

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Related PRs (per-endpoint speaker diarization, multi-repo):

• jitsi-meet: feat(transcription): add diarize flag advertised as a participant property jitsi-meet#17614
• jitsi-xmpp-extensions: feat: add diarize attribute to Colibri2Endpoint jitsi-xmpp-extensions#145
• jicoco: feat: add diarize field to mediajson Start event #238
• jicofo: feat: signal per-endpoint diarize to the bridge via Colibri2 jicofo#1291
• jitsi-videobridge: feat: set per-endpoint diarize on the transcriber start event jitsi-videobridge#2431
• opus-transcriber-proxy: feat: per-endpoint diarization via the start event opus-transcriber-proxy#106

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 34.61%. Comparing base (1e2216b) to head (8cc60d2).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #238      +/-   ##
============================================
+ Coverage     34.56%   34.61%   +0.04%     
  Complexity      162      162              
============================================
  Files            36       36              
  Lines          1380     1381       +1     
  Branches        140      140              
============================================
+ Hits            477      478       +1     
  Misses          870      870              
  Partials         33       33              

Files with missing lines	Coverage Δ
...n/src/main/kotlin/org/jitsi/mediajson/MediaJson.kt	98.48% <100.00%> (+0.02%)	⬆️

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