Derive matrix_synapse_experimental_features_msc4143_enabled from the configured transports

The RTC transports API was enabled based on matrix_rtc_enabled, while the
transports it serves were populated based on matrix_livekit_jwt_service_enabled.
Enabling the LiveKit JWT service on its own (say, when pointing it at an
externally hosted LiveKit server) therefore configured a transport but left
the API which announces it switched off.

Gating the feature on matrix_synapse_matrix_rtc_transports instead keeps the
API and its payload in sync, and makes the empty-transports case that the
variable's own documentation warns about unreachable.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

group_vars/matrix_servers

@@ -5195,8 +5195,6 @@ matrix_synapse_experimental_features_msc4108_enabled: "{{ matrix_authentication_
 
 matrix_synapse_delayed_events_enabled: "{{ matrix_rtc_enabled }}"
 
-matrix_synapse_experimental_features_msc4143_enabled: "{{ matrix_rtc_enabled }}"
-
 matrix_synapse_matrix_rtc_transports_livekit_url: "{{ matrix_livekit_jwt_service_public_url if matrix_livekit_jwt_service_enabled else '' }}"
 
 matrix_synapse_experimental_features_msc4222_enabled: "{{ matrix_rtc_enabled }}"

roles/custom/matrix-synapse/defaults/main.yml

@@ -1459,12 +1459,12 @@ matrix_synapse_max_event_delay_duration: 24h
 #
 # This is used by MatrixRTC clients to discover the unstable RTC transports API.
 #
-# When enabling this, make sure that `matrix_synapse_matrix_rtc_transports` is not empty.
-# Serving the RTC transports API without any transports configured breaks MatrixRTC (Element Call) for clients which consult this API.
+# This follows `matrix_synapse_matrix_rtc_transports`, because serving the RTC transports API without any transports configured
+# breaks MatrixRTC (Element Call) for clients which consult this API.
 # Such clients treat the API response as authoritative and do not fall back to the `org.matrix.msc4143.rtc_foci` property in the client well-known.
 #
 # See https://github.com/matrix-org/matrix-spec-proposals/pull/4143
-matrix_synapse_experimental_features_msc4143_enabled: false
+matrix_synapse_experimental_features_msc4143_enabled: "{{ matrix_synapse_matrix_rtc_transports | length > 0 }}"
 
 # Controls the `matrix_rtc.transports` setting - the list of MatrixRTC transports advertised via the RTC transports API (MSC4143).
 #