fix(core): Perform multi-main leader check against key ID (#7964)

## Summary
Current leader check is based on key presence and multi-main instance
type, which can give rise to [this edge
case](https://n8nio.slack.com/archives/C04B1GZ4T0U/p1702025497086379?thread_ts=1701962808.817579&cid=C04B1GZ4T0U)
where leader fails to realize they lost leadership to a former follower.
PR performs the check against the specific key ID instead to prevent
this.

...

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


## Issues fixed
Include links to Github issue or Community forum post or **Linear
ticket**:
> Important in order to close automatically and provide context to
reviewers

...


## Review / Merge checklist
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automatically goes into the changelog. Use `(no-changelog)` otherwise.**
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- [ ] [Docs updated](https://github.com/n8n-io/n8n-docs) or follow-up
ticket created.
- [ ] Tests included.
> A bug is not considered fixed, unless a test is added to prevent it
from happening again. A feature is not complete without tests.
  >
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or [deploy test
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packages/cli/src/services/orchestration/main/MultiMainSetup.ee.ts

@@ -63,41 +63,45 @@ export class MultiMainSetup extends SingleMainSetup {
 	}
 
 	private async checkLeader() {
-		if (!this.redisPublisher.redisClient) return;
-
 		const leaderId = await this.redisPublisher.get(this.leaderKey);
 
-		if (!leaderId) {
-			this.logger.debug('Leadership vacant, attempting to become leader...');
-			await this.tryBecomeLeader();
+		if (leaderId === this.id) {
+			this.logger.debug(`[Instance ID ${this.id}] Leader is this instance`);
+
+			await this.redisPublisher.setExpiration(this.leaderKey, this.leaderKeyTtl);
 
 			return;
 		}
 
-		if (this.isLeader) {
-			this.logger.debug(`Leader is this instance "${this.id}"`);
+		if (leaderId && leaderId !== this.id) {
+			this.logger.debug(`[Instance ID ${this.id}] Leader is other instance "${leaderId}"`);
 
-			await this.redisPublisher.setExpiration(this.leaderKey, this.leaderKeyTtl);
-		} else {
-			this.logger.debug(`Leader is other instance "${leaderId}"`);
+			if (config.getEnv('multiMainSetup.instanceType') === 'leader') {
+				this.emit('leadershipChange', leaderId); // stop triggers, pruning, etc.
+
+				config.set('multiMainSetup.instanceType', 'follower');
+			}
+
+			return;
+		}
+
+		if (!leaderId) {
+			this.logger.debug(
+				`[Instance ID ${this.id}] Leadership vacant, attempting to become leader...`,
+			);
 
 			config.set('multiMainSetup.instanceType', 'follower');
+
+			await this.tryBecomeLeader();
 		}
 	}
 
 	private async tryBecomeLeader() {
-		if (
-			config.getEnv('multiMainSetup.instanceType') === 'leader' ||
-			!this.redisPublisher.redisClient
-		) {
-			return;
-		}
-
 		// this can only succeed if leadership is currently vacant
 		const keySetSuccessfully = await this.redisPublisher.setIfNotExists(this.leaderKey, this.id);
 
 		if (keySetSuccessfully) {
-			this.logger.debug(`Leader is now this instance "${this.id}"`);
+			this.logger.debug(`[Instance ID ${this.id}] Leader is now this instance`);
 
 			config.set('multiMainSetup.instanceType', 'leader');