Add regression test for grouped PR with one failing metadata cascade

Verifies that when one dependency's metadata cascade raises in a
multi-dependency PR, the other dependency's metadata is unaffected
and the overall PR message still renders.

Author: yeikel

common/spec/dependabot/pull_request_creator/message_builder_spec.rb

@@ -1950,6 +1950,27 @@ def commits_details(base:, head:)
             )
         end
 
+        context "when one dependency's metadata cascade raises" do
+          before do
+            stub_request(
+              :get,
+              "https://api.github.com/repos/gocardless/statesman/contents/CHANGELOG.md?ref=master"
+            ).to_raise(SocketError, "Connection refused - connect(2) for \"api.github.com\" port 443")
+          end
+
+          it "still renders the PR message with the unaffected dependency's metadata" do
+            expect(pr_message).to include(
+              "Bumps [business](https://github.com/gocardless/business) " \
+              "and [statesman](https://github.com/gocardless/statesman). " \
+              "These dependencies needed to be updated together."
+            )
+            expect(pr_message).to include("Updates `business` from 1.4.0 to 1.5.0")
+            expect(pr_message).to include("business's changelog")
+            expect(pr_message).to include("Updates `statesman` from 1.6.0 to 1.7.0")
+            expect(pr_message).not_to include("statesman's changelog")
+          end
+        end
+
         context "when dealing with a property dependency (e.g., with Maven)" do
           before do
             statesman_repo_url =