feat(core): Add multi-main setup debug endpoint (no-changelog) (#7991)

## Summary
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Adi's idea here to help diagnose:
https://n8nio.slack.com/archives/C069KJBJ8HE/p1702300349277609?thread_ts=1702299930.728029&cid=C069KJBJ8HE
...

#### How to test the change:
1. ...


## Issues fixed
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packages/cli/src/controllers/debug.controller.ts

@@ -0,0 +1,35 @@
+import { Service } from 'typedi';
+import { Get, RestController } from '@/decorators';
+import { ActiveWorkflowRunner } from '@/ActiveWorkflowRunner';
+import { MultiMainSetup } from '@/services/orchestration/main/MultiMainSetup.ee';
+import { WorkflowRepository } from '@/databases/repositories/workflow.repository';
+import { In } from 'typeorm';
+
+@RestController('/debug')
+@Service()
+export class DebugController {
+	constructor(
+		private readonly multiMainSetup: MultiMainSetup,
+		private readonly activeWorkflowRunner: ActiveWorkflowRunner,
+		private readonly workflowRepository: WorkflowRepository,
+	) {}
+
+	@Get('/multi-main-setup')
+	async getMultiMainSetupDetails() {
+		const leaderKey = await this.multiMainSetup.fetchLeaderKey();
+
+		const activeWorkflows = await this.workflowRepository.find({
+			select: ['id', 'name'],
+			where: { id: In(this.activeWorkflowRunner.allActiveInMemory()) },
+		});
+
+		const activationErrors = await this.activeWorkflowRunner.getAllWorkflowActivationErrors();
+
+		return {
+			instanceId: this.multiMainSetup.instanceId,
+			leaderKey,
+			activeWorkflows,
+			activationErrors,
+		};
+	}
+}