Improve the unauthorized error message after cutoff

doc/user/gitlab_duo_chat/troubleshooting.md

@@ -70,6 +70,15 @@ This error occurs when you try to access a tool of GitLab Duo Chat that is not b
 
 Ensure your [GitLab Duo subscription tier](https://about.gitlab.com/gitlab-duo/#pricing) includes the selected tool.
 
+## `Error M3006`
+
+You might get an error that states
+`I'm sorry, you don't have the GitLab Duo subscription required to use Duo Chat. Please contact your administrator. Error code: M3006`.
+
+This error occurs when GitLab Duo Chat is not included in your GitLab Duo subscription.
+
+Ensure your [GitLab Duo subscription tier](https://about.gitlab.com/gitlab-duo/#pricing) includes GitLab Duo Chat.
+
 ## `Error M4000`
 
 You might get an error that states

ee/lib/gitlab/llm/chain/agents/single_action_executor.rb

@@ -110,6 +110,15 @@ def error_answer(error)
                   error_code: "A1004"
                 )
               end
+            when Gitlab::AiGateway::ForbiddenError
+              Answer.error_answer(
+                error: error,
+                context: context,
+                content: _("I'm sorry, you don't have the GitLab Duo subscription required " \
+                  "to use Duo Chat. Please contact your administrator."),
+                source: "chat_v2",
+                error_code: "M3006"
+              )
             else
               Answer.error_answer(
                 error: error,

ee/spec/lib/gitlab/llm/chain/agents/single_action_executor_spec.rb

@@ -412,6 +412,26 @@
       end
     end
 
+    context "when forbidden error is raised" do
+      let(:error) { ::Gitlab::AiGateway::ForbiddenError.new }
+
+      before do
+        allow(Gitlab::ErrorTracking).to receive(:track_exception)
+
+        allow_next_instance_of(Gitlab::Duo::Chat::StepExecutor) do |react_agent|
+          allow(react_agent).to receive(:step).and_raise(error)
+        end
+      end
+
+      it "returns an error" do
+        expect(answer.is_final).to eq(true)
+        expect(answer.content).to include("I'm sorry, you don't have the GitLab Duo subscription required " \
+          "to use Duo Chat. Please contact your administrator.")
+        expect(answer.error_code).to include("M3006")
+        expect(Gitlab::ErrorTracking).to have_received(:track_exception).with(error)
+      end
+    end
+
     context "when eof error is raised" do
       let(:error) { EOFError.new }
 

locale/gitlab.pot

@@ -27500,6 +27500,9 @@ msgstr ""
 msgid "I'm sorry, I couldn't respond in time. Please try again."
 msgstr ""
+msgid "I'm sorry, you don't have the GitLab Duo subscription required to use Duo Chat. Please contact your administrator."
+msgstr ""
+
 msgid "I'm sorry, you've entered too many prompts. Please run /clear or /reset before asking the next question."
 msgstr ""