fix: await async component registration during startup

Replace fire-and-forget subscribe() with centralized block-and-error handling so async tool, prompt, and resource registration completes before the server starts.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: codeboyzhou

src/main/java/com/github/thought2code/mcp/annotated/server/component/AbstractDualModeComponentRegistrar.java

@@ -1,13 +1,15 @@
 package com.github.thought2code.mcp.annotated.server.component;
 
 import com.github.thought2code.mcp.annotated.McpApplicationContext;
+import com.github.thought2code.mcp.annotated.exception.McpServerComponentRegistrationException;
 import io.modelcontextprotocol.server.McpAsyncServer;
 import io.modelcontextprotocol.server.McpSyncServer;
 import java.lang.annotation.Annotation;
 import java.lang.reflect.Method;
 import java.util.Set;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
+import reactor.core.publisher.Mono;
 
 /**
  * Template base class for component registrars that support both sync and async server modes.
@@ -60,8 +62,9 @@ public abstract class AbstractDualModeComponentRegistrar<S, A> implements McpCom
    *
    * @param server target async server
    * @param specification specification to register
+   * @return registration completion signal
    */
-  protected abstract void addAsyncSpecification(McpAsyncServer server, A specification);
+  protected abstract Mono<Void> addAsyncSpecification(McpAsyncServer server, A specification);
 
   /**
    * @param specification registered sync specification
@@ -95,9 +98,38 @@ public final void register(McpAsyncServer server, McpApplicationContext context)
       final String annotationTypeName = annotationType().getSimpleName();
       log.debug("Registering async {} method: {}", annotationTypeName, method.toGenericString());
       A specification = createAsyncSpecification(method, context);
-      addAsyncSpecification(server, specification);
       final String specificationName = asyncSpecificationName(specification);
+      Mono<Void> registration = addAsyncSpecification(server, specification);
+      awaitAsyncRegistration(registration, annotationTypeName, specificationName);
       log.debug("Async {} {} registered successfully", annotationTypeName, specificationName);
     }
   }
+
+  /**
+   * Waits for an asynchronous component registration to complete during server startup.
+   *
+   * <p>Async MCP server APIs return a {@link Mono} that completes when registration finishes. This
+   * method blocks on that signal so registration behaves like the synchronous path: each component
+   * is fully registered before the next one is processed and before the server starts handling
+   * requests.
+   *
+   * <p>If registration fails, the error is logged and rethrown as a {@link
+   * McpServerComponentRegistrationException} with component context.
+   *
+   * @param registration registration completion signal from {@link #addAsyncSpecification}
+   * @param componentType annotation type name used in error messages (for example, {@code McpTool})
+   * @param specificationName registered component name used in error messages
+   * @throws McpServerComponentRegistrationException if registration fails
+   */
+  private void awaitAsyncRegistration(
+      Mono<Void> registration, String componentType, String specificationName) {
+    try {
+      registration.block();
+    } catch (RuntimeException e) {
+      final String message =
+          String.format("Failed to register async %s %s", componentType, specificationName);
+      log.error(message, e);
+      throw new McpServerComponentRegistrationException(message, e);
+    }
+  }
 }

src/main/java/com/github/thought2code/mcp/annotated/server/component/McpServerPrompt.java

@@ -130,9 +130,9 @@ protected void addSyncSpecification(
   }
 
   @Override
-  protected void addAsyncSpecification(
+  protected Mono<Void> addAsyncSpecification(
       McpAsyncServer server, McpServerFeatures.AsyncPromptSpecification specification) {
-    server.addPrompt(specification).subscribe();
+    return server.addPrompt(specification);
   }
 
   @Override

src/main/java/com/github/thought2code/mcp/annotated/server/component/McpServerResource.java

@@ -115,9 +115,9 @@ protected void addSyncSpecification(
   }
 
   @Override
-  protected void addAsyncSpecification(
+  protected Mono<Void> addAsyncSpecification(
       McpAsyncServer server, McpServerFeatures.AsyncResourceSpecification specification) {
-    server.addResource(specification).subscribe();
+    return server.addResource(specification);
   }
 
   @Override

src/main/java/com/github/thought2code/mcp/annotated/server/component/McpServerTool.java

@@ -143,9 +143,9 @@ protected void addSyncSpecification(
   }
 
   @Override
-  protected void addAsyncSpecification(
+  protected Mono<Void> addAsyncSpecification(
       McpAsyncServer server, McpServerFeatures.AsyncToolSpecification specification) {
-    server.addTool(specification).subscribe();
+    return server.addTool(specification);
   }
 
   @Override