Fix concurrent sendMessage race by retrying on FAIL_NON_SERIALIZED

Replace tryEmitNext (fail-fast) with emitNext + busyLooping(100ms)
in ManagedStdioClientTransport.sendMessage().

The unicast sink's SinkManySerialized wrapper returns FAIL_NON_SERIALIZED
when two threads call tryEmitNext concurrently. busyLooping retries the
CAS spin instead of immediately failing, making concurrent sends safe.

Before: 19/20 test repetitions fail
After:  20/20 pass

Author: AbbasNS

src/main/java/org/sonarsource/sonarqube/mcp/client/ManagedStdioClientTransport.java

@@ -137,11 +137,16 @@ private void handleIncomingErrors() {
     errorSink.asFlux().subscribe(stdErrorHandler);
   }
 
+  private static final Duration EMIT_TIMEOUT = Duration.ofMillis(100);
+
   @Override
   public Mono<Void> sendMessage(McpSchema.JSONRPCMessage message) {
-    return outboundSink.tryEmitNext(message).isSuccess() 
-      ? Mono.empty() 
-      : Mono.error(new RuntimeException("Failed to enqueue message for '" + serverName + "'"));
+    try {
+      outboundSink.emitNext(message, Sinks.EmitFailureHandler.busyLooping(EMIT_TIMEOUT));
+      return Mono.empty();
+    } catch (Sinks.EmissionException e) {
+      return Mono.error(new RuntimeException("Failed to enqueue message for '" + serverName + "'", e));
+    }
   }
 
   private void startInboundProcessing() {