Add createServiceMock to backend-test-utils

Exports the internal simpleMock utility as createServiceMock, allowing
plugin authors to define mock creators for their own service refs
following the same pattern as the built-in mockServices mocks.

Also updates catalog-node to use createServiceMock instead of its own
copy, and simplifies the gateway-backend test to use the mock's
.factory property directly.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Co-authored-by: Cursor <cursoragent@cursor.com>
Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

yarn.lock

@@ -5399,6 +5399,11 @@ __metadata:
     lodash: "npm:^4.17.21"
     msw: "npm:^1.0.0"
     yaml: "npm:^2.0.0"
+  peerDependencies:
+    "@backstage/backend-test-utils": "workspace:^"
+  peerDependenciesMeta:
+    "@backstage/backend-test-utils":
+      optional: true
   languageName: unknown
   linkType: soft