[red-knot] mypy_primer: pipeline improvements (#16620)

## Summary

- Add comment to explain `sed` command
- Fix double reporting of diff
- Hide (large) diffs in `<details>`

.github/workflows/mypy_primer.yaml

@@ -61,16 +61,22 @@ jobs:
           echo "Running mypy_primer"
 
           (
-            uvx --from "git+https://github.com/astral-sh/mypy_primer.git@add-red-knot-support" mypy_primer \
+            uvx --verbose --from "git+https://github.com/astral-sh/mypy_primer.git@add-red-knot-support" mypy_primer \
               --repo ruff \
               --type-checker knot \
               --old base_commit \
               --new "$GITHUB_SHA" \
               --project-selector '/(mypy_primer|black|pyp|git-revise|zipp|arrow)$' \
               --output concise \
-              --debug | tee /dev/stdout | sed -e 's/\x1b\[[0-9;]*m//g' > mypy_primer.diff
+              --debug > mypy_primer.diff
           ) || [ $? -eq 1 ]
 
+          # Output diff with ANSI color codes
+          cat mypy_primer.diff
+
+          # Remove ANSI color codes before uploading
+          sed -ie 's/\x1b\[[0-9;]*m//g' mypy_primer.diff
+
           echo ${{ github.event.number }} > pr-number
 
       - name: Upload diff

.github/workflows/mypy_primer_comment.yaml

@@ -63,9 +63,13 @@ jobs:
 
           echo '## `mypy_primer` results' >> comment.txt
           if [ -s "pr/mypy_primer_diff/mypy_primer.diff" ]; then
+            echo '<details>' >> comment.txt
+            echo '<summary>Changes were detected when running on open source projects</summary>' >> comment.txt
+            echo '' >> comment.txt
             echo '```diff' >> comment.txt
             cat pr/mypy_primer_diff/mypy_primer.diff >> comment.txt
             echo '```' >> comment.txt
+            echo '</details>' >> comment.txt
           else
             echo 'No ecosystem changes detected ✅' >> comment.txt
           fi