[DanTupBot]

Implements Dart-Code/Dart-Code#6375 for Dart-Code/Flutter (matching the changes in Dart-Code/Dart-Code):

• Adds ovsx to devDependencies.
• Adds a Publish to Open VSX step to .github/workflows/release.yml using OIDC Trusted Publishing (--trusted-publishing) and --skip-duplicate to allow safe retries.
• Prints the ovsx publish command (after verifying OIDC environment variables) when dry_run is enabled.
• Updates Print retry instructions on failure to trigger if either publish step (vsce or ovsx) succeeded before a later step failed, and aligns comments in release.yml with Dart-Code/Dart-Code.

[DanTupBot]

LGTM!

• ovsx (^1.4.0) is added to devDependencies (compatible with Node 22 used in the workflow).
• The Publish to Open VSX step matches the VS Code Marketplace step structure (including OIDC environment verification during dry_run, --trusted-publishing, and --skip-duplicate for safe retries, while omitting --pre-release so ovsx reads the flag from the .vsix manifest).
• Failure retry instructions properly check both publish_vscode and publish_openvsx outcomes, and .github/workflows/release.yml is kept in sync with Dart-Code/Dart-Code.