Attach a ready-to-apply patch when the amalgamation check fails (#5229)

When a PR is not amalgamated/formatted, the astyle version friction (see
the recurring blocker across many PRs) means contributors often struggle
to reproduce the exact fix locally. The check now regenerates the
amalgamation and formatting, captures the difference as a patch, and
uploads it as the `amalgamation-patch` artifact. The failure comment
links to that artifact and tells contributors to run
`git apply amalgamation.patch`, so they no longer need to install the
pinned astyle version themselves.

The pass/fail verdict is unchanged: the same PRs fail as before, and a
correctly amalgamated PR uploads nothing and passes.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/comment_check_amalgamation.yml

@@ -24,6 +24,7 @@ jobs:
           egress-policy: audit
 
       - name: 'Download artifact'
+        id: download
         uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9.0.0
         with:
           script: |
@@ -43,6 +44,9 @@ jobs:
             });
             var fs = require('fs');
             fs.writeFileSync('${{github.workspace}}/pr.zip', Buffer.from(download.data));
+
+            var hasPatch = artifacts.data.artifacts.some((artifact) => artifact.name == "amalgamation-patch");
+            core.setOutput('has_patch', String(hasPatch));
       - run: unzip pr.zip
 
       - name: 'Comment on PR'
@@ -70,12 +74,20 @@ jobs:
                 break
               }
             }
+            const hasPatch = '${{ steps.download.outputs.has_patch }}' === 'true';
+            const runUrl = '${{ github.event.workflow_run.html_url }}';
+
             await github.rest.issues.createComment({
               issue_number: issue_number,
               owner: context.repo.owner,
               repo: context.repo.repo,
-              body: '## 🔴 Amalgamation check failed! 🔴\nThe source code has not been amalgamated.'
-                    + (first ? ' @' + author + ' Please read and follow the [Contribution Guidelines]'
+              body: '## 🔴 Amalgamation check failed! 🔴\nThe source code has not been amalgamated and/or formatted correctly.'
+                    + (hasPatch ? '\n\n📎 A ready-to-apply patch is attached to the [failed workflow run](' + runUrl + ') as the `amalgamation-patch` artifact.'
+                                  + ' Download it, then apply it locally from the repository root with:'
+                                  + '\n\n```shell\ngit apply amalgamation.patch\n```\n\n'
+                                  + 'This does not require installing astyle yourself.'
+                                : '')
+                    + (first ? '\n\n@' + author + ' Please read and follow the [Contribution Guidelines]'
                                + '(https://github.com/nlohmann/json/blob/develop/.github/CONTRIBUTING.md#files-to-change).'
                              : '')
               })