MqttFrameMetadata

MQTT topic/qos/retained metadata carried in SocketMessage.contentType rather than as new fields on SocketMessage -- see the design spec's "MQTT capture reuses the socket pipeline" section. The encoding is hand-rolled and dependency-free (no JSON/URL-encoding library pulled into sdk:socket just for this): "application/mqtt; topic=<enc>; qos=<n>; retained=<bool>", where <enc> escapes only the three characters that would otherwise break this format's own delimiters.

Every parser here is fail-soft: malformed input, a non-MQTT content type, or null all return null rather than throw, matching this module's fail-open recording discipline.