fix(delegation): guard concurrent rollback pop and add transition tests

Author: edelauna

src/core/webview/ClineProvider.ts

@@ -3552,7 +3552,11 @@ export class ClineProvider
 				}`,
 			)
 			try {
-				await this.removeClineFromStack({ skipDelegationRepair: true })
+				// Only pop the stack if the child we just created is still on top.
+				// A concurrent delegation could have pushed another child since we created ours.
+				if (this.getCurrentTask()?.taskId === child.taskId) {
+					await this.removeClineFromStack({ skipDelegationRepair: true })
+				}
 			} catch (cleanupError) {
 				this.log(
 					`[delegateParentAndOpenChild] Failed to close paused child ${child.taskId} during rollback: ${