fix(update-flathub-repo): Improve Flathub metainfo changelog insertion

Set CHANGELOG from the release body and robustly insert it into the generated .metainfo.xml. Escape XML-sensitive characters (&, <, >), disable patsub_replacement to treat replacement text literally, and read the full metainfo file while preserving newlines. Fail with a clear error if the expected <!-- changelog --> placeholder is missing. Replace the placeholder using Bash string substitution and write the updated contents back (replacing the previous sed-based approach) to avoid issues with sed and multiline input.

Author: ReenigneArcher

.github/workflows/__call-update-flathub-repo.yml

@@ -130,15 +130,34 @@ jobs:
         if: >-
           steps.check-label.outputs.hasTopic == 'true' &&
           steps.check-release.outputs.isLatestRelease == 'true'
+        env:
+          CHANGELOG: ${{ github.event.release.body }}
         run: |
+          # Set the metainfo file and placeholder generated by the release asset.
           xml_file="flathub/${{ env.FLATHUB_PKG }}/${{ env.FLATHUB_PKG }}.metainfo.xml"
+          placeholder="<!-- changelog -->"
+
+          # Treat replacement text literally during Bash parameter substitutions.
+          shopt -u patsub_replacement 2>/dev/null || true
+
+          # Escape XML-sensitive characters from the release body.
+          changelog="${CHANGELOG:-}"
+          changelog="${changelog//&/&amp;}"
+          changelog="${changelog//</&lt;}"
+          changelog="${changelog//>/&gt;}"
+
+          # Read the full metainfo file, preserving newlines.
+          IFS= read -r -d '' contents < "$xml_file" || true
 
-          # Extract release information
-          changelog="${{ github.event.release.body }}" && changelog="${changelog//&/&amp;}" && \
-            changelog="${changelog//</&lt;}" && changelog="${changelog//>/&gt;}"
+          # Fail clearly if the generated metainfo file no longer has the expected marker.
+          if [[ "$contents" != *"$placeholder"* ]]; then
+            echo "$placeholder not found in $xml_file"
+            exit 1
+          fi
 
-          # Replace changelog placeholder with actual changelog
-          sed -i "s|<!-- changelog -->|$changelog|g" "$xml_file"
+          # Replace the placeholder with the escaped changelog and write the file back.
+          contents="${contents//$placeholder/$changelog}"
+          printf '%s' "$contents" > "$xml_file"
 
       - name: Update submodule
         if: >-