fix: Keep delegated handlers when unregistering actions (#6395)

## Explanation

Keep delegated handlers around even when unregistering actions. This
helps with the resiliency of the messenger system in case actions are
unregistered and registered elsewhere in the chain of delegations. This
is also very useful for testing.

Additionally adjusts the error message thrown when a delegated action
handler cannot be found to match an undelegated one.

Author: FrederikBolding

packages/messenger/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- Keep delegated handlers when unregistering actions ([#6395](https://github.com/MetaMask/core/pull/6395))
+
 ## [0.1.0]
 
 ### Added

packages/messenger/src/Messenger.test.ts

@@ -1264,31 +1264,6 @@ describe('Messenger', () => {
         actions: ['Source:getLength'],
       });
 
-      expect(() => delegatedMessenger.call('Source:getLength', 'test')).toThrow(
-        `Cannot call 'Source:getLength', action not registered.`,
-      );
-    });
-
-    it('unregisters delegated action handlers when action is unregistered', () => {
-      type ExampleAction = {
-        type: 'Source:getLength';
-        handler: (input: string) => number;
-      };
-      const sourceMessenger = new Messenger<'Source', ExampleAction, never>({
-        namespace: 'Source',
-      });
-      const delegatedMessenger = new Messenger<
-        'Destination',
-        ExampleAction,
-        never
-      >({ namespace: 'Destination' });
-      sourceMessenger.delegate({
-        messenger: delegatedMessenger,
-        actions: ['Source:getLength'],
-      });
-
-      sourceMessenger.unregisterActionHandler('Source:getLength');
-
       expect(() => delegatedMessenger.call('Source:getLength', 'test')).toThrow(
         `A handler for Source:getLength has not been registered`,
       );

packages/messenger/src/Messenger.ts

@@ -356,14 +356,6 @@ export class Messenger<
     actionType: ActionType,
   ) {
     this.#actions.delete(actionType);
-    const delegationTargets = this.#actionDelegationTargets.get(actionType);
-    if (!delegationTargets) {
-      return;
-    }
-    for (const messenger of delegationTargets) {
-      messenger._internalUnregisterDelegatedActionHandler(actionType);
-    }
-    this.#actionDelegationTargets.delete(actionType);
   }
 
   /**
@@ -763,7 +755,7 @@ export class Messenger<
           | undefined;
         if (!actionHandler) {
           throw new Error(
-            `Cannot call '${actionType}', action not registered.`,
+            `A handler for ${actionType} has not been registered`,
           );
         }
         return actionHandler(...args);