Emit OnMessageClause as its message reference, not a quoted identifier

An OnMessageClause's id is a synthetic "keyword path" string (e.g.
"command Go"). After Identifier.format began single-quoting non-bare
names, openDef rendered the clause as `on 'command Go'`, which does not
re-parse. Emit the clause's message reference (msg.format) instead, which
is the round-trippable source form; all other synthetic ids are bare
literals and unaffected.

Fixes JsonInputTest reference-carrying-statements round-trip on JVM, JS,
and Native.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: reid-spencer

passes/shared/src/main/scala/com/ossuminc/riddl/passes/prettify/RiddlFileEmitter.scala

@@ -37,7 +37,13 @@ case class RiddlFileEmitter(url: URL)(using PlatformContext) extends FileBuilder
     definition: Definition,
     withBrace: Boolean = true
   ): this.type = {
-    addIndent(s"${keyword(definition)} ${definition.id.format} is ")
+    // An OnMessageClause's id is a synthetic "keyword path" string (e.g.
+    // "command Go"); emit the message reference itself, which is the
+    // round-trippable source form, rather than quoting it as an identifier.
+    val name = definition match
+      case omc: OnMessageClause => omc.msg.format
+      case _                    => definition.id.format
+    addIndent(s"${keyword(definition)} $name is ")
     if withBrace then
       if definition.isEmpty then add("{ ??? }").nl
       else add("{").nl.incr