fix(api): add clearAllMocks and explicit return type to GroupsModule

- Add jest.clearAllMocks() in beforeEach of both spec files to prevent
  call-history state from leaking between tests as the suites grow
- Define GroupSummary interface and use it as explicit return type for
  GroupsService.list() to satisfy TypeScript strict typing requirements

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/api/src/modules/groups/groups.service.ts

@@ -1,11 +1,20 @@
 import { Injectable } from '@nestjs/common';
 import { PrismaService } from '../../prisma/prisma.service';
 
+interface GroupSummary {
+  id: string;
+  name: string;
+  platform: string;
+  platformId: string;
+  isActive: boolean;
+  accountId: string | null;
+}
+
 @Injectable()
 export class GroupsService {
   constructor(private readonly prisma: PrismaService) {}
 
-  list() {
+  list(): Promise<GroupSummary[]> {
     return this.prisma.group.findMany({
       orderBy: { name: 'asc' },
       select: { id: true, name: true, platform: true, platformId: true, isActive: true, accountId: true },