fix(ci): validate frozen content with MDX parser after sanitization

Run tools/check-docs-mdx-parse on each version's sanitized content so
any escaping gap is caught before publish rather than at fern generate.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: pdmack

.github/workflows/fern-docs-preview-build.yml

@@ -89,6 +89,7 @@ jobs:
                   }
                 ' "$f" > "${f}.tmp" && mv "${f}.tmp" "$f"
               done
+              tools/check-docs-mdx-parse "fern/versions/${version}-content"
               echo "Extracted docs from $version"
             else
               echo "::warning::Tag $version not found — skipping content checkout"

.github/workflows/publish-fern-docs.yml

@@ -205,6 +205,7 @@ jobs:
                   }
                 ' "$f" > "${f}.tmp" && mv "${f}.tmp" "$f"
               done
+              tools/check-docs-mdx-parse "fern/versions/${version}-content"
               echo "Extracted docs from $version"
             else
               echo "::error::Tag $version not found — cannot pin frozen docs content"