Fix service validation tag metadata (#7509)

fix #5933

Author: timotheeguerin

.chronus/changes/fix-service-validation-tag-metadata-2025-4-29-22-43-37.md

@@ -0,0 +1,8 @@
+---
+# Change versionKind to one of: internal, fix, dependencies, feature, deprecation, breaking
+changeKind: fix
+packages:
+  - "@typespec/openapi"
+---
+
+Fix `@tagMetadata` decorator emitting error when incorrectly not finding `@service` decorator

packages/openapi/src/decorators.ts

@@ -1,5 +1,4 @@
 import {
-  $service,
   compilerAssert,
   DecoratorContext,
   getDoc,
@@ -282,7 +281,13 @@ export const tagMetadataDecorator: TagMetadataDecorator = (
   tagMetadata: TagMetadata,
 ) => {
   // Check if the namespace is a service namespace
-  if (!entity.decorators.some((decorator) => decorator.decorator === $service)) {
+  if (
+    !entity.decorators.some(
+      (decorator) =>
+        decorator.definition?.name === "@service" &&
+        decorator.definition?.namespace.name === "TypeSpec",
+    )
+  ) {
     reportDiagnostic(context.program, {
       code: "tag-metadata-target-service",
       format: {