Fix terminal detach session

Fixes #137204

src/vs/platform/terminal/node/ptyService.ts

@@ -234,7 +234,7 @@ export class PtyService extends Disposable implements IPtyService {
 	}
 
 	async detachFromProcess(id: number): Promise<void> {
-		this._throwIfNoPty(id).detach();
+		return this._throwIfNoPty(id).detach();
 	}
 
 	async reduceConnectionGraceTime(): Promise<void> {

src/vs/workbench/contrib/terminal/browser/terminalInstance.ts

@@ -916,7 +916,10 @@ export class TerminalInstance extends Disposable implements ITerminalInstance {
 	}
 
 	async detachFromProcess(): Promise<void> {
+		// Detach the process and dispose the instance, without the instance dispose the terminal
+		// won't go away
 		await this._processManager.detachFromProcess();
+		this.dispose();
 	}
 
 	focus(force?: boolean): void {

src/vs/workbench/contrib/terminal/browser/terminalProcessManager.ts

@@ -167,6 +167,7 @@ export class TerminalProcessManager extends Disposable implements ITerminalProce
 
 	async detachFromProcess(): Promise<void> {
 		await this._process?.detach?.();
+		this._process = null;
 	}
 
 	async createProcess(