Updated error message when VR MR suggestions can’t be created

Messaging is now more specific when errors occur when attempting to resolve a vulnerability with AI in a merge request.

Changelog: changed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/168986
EE: true

ee/lib/gitlab/llm/completions/resolve_vulnerability.rb

@@ -11,7 +11,8 @@ class ResolveVulnerability < Gitlab::Llm::Completions::Base
                                    'AI resolution to a merge request.'
         MR_LINK_ERROR = 'An error occurred while attempting to link the MR to the vulnerability.'
         RESPONSE_FAILURE_ERROR = 'Response from AI is unreadable. Please try again.'
-        MERGE_REQUEST_SUGGESTION_ERROR = 'An error occurred while attempting to create the merge request suggestion.'
+        MERGE_REQUEST_SUGGESTION_ERROR = 'Resolution target could not be found in the merge request, ' \
+                                    'unable to create suggestion.'
         START_CODE = "<fixed_code>\n"
         END_CODE = "</fixed_code>"
         START_ANALYSIS = "<analysis>\n"

lib/gitlab/diff/merge_request_suggestion.rb

@@ -116,6 +116,8 @@ def latest_merge_request_diff
       strong_memoize_attr :latest_merge_request_diff
       def position
+        raise TargetLineNotFound if suggestion_target_line.nil?
+
         {
           position_type: "text",
           old_path: @path,

spec/lib/gitlab/diff/merge_request_suggestion_spec.rb

@@ -25,8 +25,9 @@
     end
     let_it_be(:diff) { File.read(File.join(fixtures_folder, 'input.diff')) }
-    subject(:attributes_hash) { described_class.new(diff, filepath, merge_request).note_attributes_hash }
+    let(:mr_suggestion) { described_class.new(diff, filepath, merge_request) }
+    subject(:attributes_hash) { mr_suggestion.note_attributes_hash }
     before do
       merge_request.reload
@@ -61,5 +62,12 @@
         expect { attributes_hash }.to raise_exception(described_class::TargetLineNotFound)
       end
     end
+
+    context 'when suggestion_target_line is nil' do
+      it 'raises an error' do
+        expect(mr_suggestion).to receive(:suggestion_target_line).and_return(nil)
+        expect { attributes_hash }.to raise_exception(described_class::TargetLineNotFound)
+      end
+    end
   end
 end