fix(catalog-backend-module-gitlab): use correct GitLab API parameter 'topic' instead of 'topics'

The GitLab Projects API expects the query parameter 'topic' (singular),
but the GitlabDiscoveryEntityProvider was passing 'topics' (plural),
causing project filtering by topics to silently fail.

- Updated ListProjectOptions interface to use 'topic' instead of 'topics'
- Updated GitlabDiscoveryEntityProvider to send 'topic' parameter
- Added regression test verifying topic parameter is passed correctly
- Fixed template literal formatting in client.ts

Fixes #33110

Signed-off-by: AmateurMind <suhail17mohammad@gmail.com>

.changeset/fix-gitlab-topic-filter.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-catalog-backend-module-gitlab': patch
+---
+
+Fixed GitLab project topic filtering by using correct API parameter 'topic' instead of 'topics'

plugins/catalog-backend-module-gitlab/src/lib/client.ts

@@ -41,7 +41,7 @@ interface ListProjectOptions extends CommonListOptions {
   archived?: boolean;
   group?: string;
   membership?: boolean;
-  topics?: string;
+  topic?: string;
   last_activity_after?: string;
 }
 

plugins/catalog-backend-module-gitlab/src/providers/GitlabDiscoveryEntityProvider.test.ts

@@ -827,3 +827,67 @@ describe('GitlabDiscoveryEntityProvider - simple parameter', () => {
     });
   });
 });
+
+describe('GitlabDiscoveryEntityProvider - topic parameter', () => {
+  it('should pass topic (singular) when topics (plural) is configured', async () => {
+    const config = new ConfigReader({
+      integrations: {
+        gitlab: [
+          {
+            host: 'example.com',
+            apiBaseUrl: 'https://example.com/api/v4',
+            token: 'test-token',
+          },
+        ],
+      },
+      catalog: {
+        providers: {
+          gitlab: {
+            'test-id': {
+              host: 'example.com',
+              group: 'test-group',
+              topics: ['topic1', 'topic2'],
+            },
+          },
+        },
+      },
+    });
+
+    const schedule = new PersistingTaskRunner();
+    const entityProviderConnection: EntityProviderConnection = {
+      applyMutation: jest.fn(),
+      refresh: jest.fn(),
+    };
+
+    const provider = GitlabDiscoveryEntityProvider.fromConfig(config, {
+      logger,
+      schedule,
+    })[0];
+
+    // Mock the GitLabClient listProjects method to verify parameters
+    const mockListProjects = jest.fn().mockResolvedValue({
+      items: [],
+      nextPage: undefined,
+    });
+
+    (provider as any).gitLabClient.listProjects = mockListProjects;
+
+    await provider.connect(entityProviderConnection);
+    await provider.refresh(logger);
+
+    expect(mockListProjects).toHaveBeenCalledWith({
+      group: 'test-group',
+      page: undefined,
+      per_page: 50,
+      archived: false,
+      topic: 'topic1,topic2', // Correct singular 'topic' parameter
+      simple: true,
+    });
+    // Verify the old incorrect 'topics' key is NOT present
+    expect(mockListProjects).not.toHaveBeenCalledWith(
+      expect.objectContaining({
+        topics: expect.anything(),
+      }),
+    );
+  });
+});

plugins/catalog-backend-module-gitlab/src/providers/GitlabDiscoveryEntityProvider.ts

@@ -383,7 +383,7 @@ export class GitlabDiscoveryEntityProvider implements EntityProvider {
         per_page: 50,
         ...(!this.config.includeArchivedRepos && { archived: false }),
         ...(this.config.membership && { membership: true }),
-        ...(this.config.topics && { topics: this.config.topics }),
+        ...(this.config.topics && { topic: this.config.topics }),
         // Only use simple=true when we don't need to skip forked repos.
         // The simple=true parameter reduces response size by returning fewer fields,
         // but it excludes the 'forked_from_project' field which is required for fork detection.