[key-manager] add ClearKek() to remove KEK when no longer needed (#13072)

This commit introduces the `KeyManager::ClearKek()` method, which clears
the `Kek` and resets the `mIsKekSet` flag.

The KEK is a temporary key used during the commissioning and entrust
phases. To improve security and key hygiene, this commit updates the
`Joiner` and `JoinerRouter` to explicitly clear the KEK once these
operations have concluded.

Specifically:
- `Joiner::Finish()` clears the KEK when finishing in `kStateEntrust`
  or `kStateJoined`.
- `JoinerRouter::HandleJoinerEntrustResponse()` clears the KEK
  immediately upon handling the entrust response, before scheduling
  any delayed entrusts (which set their own KEK from metadata).

src/core/meshcop/joiner.cpp

@@ -184,6 +184,7 @@ void Joiner::Finish(Error aError)
     case kStateJoined:
         Get<Tmf::SecureAgent>().Disconnect();
         mTimer.Stop();
+        Get<KeyManager>().ClearKek();
 
         OT_FALL_THROUGH;
 

src/core/meshcop/joiner_router.cpp

@@ -302,6 +302,8 @@ exit:
 
 void JoinerRouter::HandleJoinerEntrustResponse(Coap::Msg *aMsg, Error aResult)
 {
+    Get<KeyManager>().ClearKek();
+
     SendDelayedJoinerEntrust();
 
     VerifyOrExit(aResult == kErrorNone && aMsg != nullptr);

src/core/thread/key_manager.cpp

@@ -516,6 +516,12 @@ void KeyManager::SetKek(const Kek &aKek)
     mIsKekSet        = true;
 }
 
+void KeyManager::ClearKek(void)
+{
+    mKek.Clear();
+    mIsKekSet = false;
+}
+
 void KeyManager::SetSecurityPolicy(const SecurityPolicy &aSecurityPolicy)
 {
     SecurityPolicy newPolicy = aSecurityPolicy;

src/core/thread/key_manager.hpp

@@ -485,6 +485,11 @@ public:
      */
     void SetKek(const uint8_t *aKekBytes) { SetKek(*reinterpret_cast<const Kek *>(aKekBytes)); }
 
+    /**
+     * Clears the KEK.
+     */
+    void ClearKek(void);
+
     /**
      * Returns the current KEK Frame Counter value.
      *