fix: update publishing workflow to support Open VSX Registry

[mvgadagi]

Summary

This PR enables publishing the VS Code extension to the Open VSX Registry alongside the existing VS Code Marketplace publishing workflow. It updates the secure publish workflow, adds package integrity checks, and updates the contributor documentation accordingly.

Changes

.github/workflows/vscode-extension-secure-publish.yml

• Default target registries expanded: Changed the target_registries workflow input default from "vscode-marketplace" to "vscode-marketplace,open-vsx", so Open VSX publishing is enabled by default.
• Open VSX job trigger condition updated: The publish-to-open-vsx job now also runs on release events (github.event_name == 'release'), not just when manually triggered with open-vsx in the target registries input.
• Dynamic environment URL: Replaced the hardcoded extension URL with a dynamic one using ${{ env.EXTENSION_NAME }} instead of the literal prompt-registry.
• Additional package integrity check: Added a check to verify the .vsix package file exists before attempting to publish, failing the job if the file is missing.
• Comment cleanup: Updated inline comments for clarity.

docs/contributor-guide/releasing.md

• Updated the release documentation to mention that publishing now targets both VS Code Marketplace and Open VSX Registry.
• Documented the required publishing secrets: VSCODE_MARKETPLACE_TOKEN and OPEN_VSX_TOKEN.

Type of Change

• 📝 Documentation update
• 🔧 Configuration/build changes to enable openvsx publish

Testing

• Manual testing completed on the workflow dispatch and release triggers.
• Verified that the Open VSX publish job correctly activates on release events.

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have made corresponding changes to the documentation
• My changes generate no new warnings or errors

[wherka-ama]

/LGTM thanks a lot @mvgadagi ❤️

[gblanc-1a]

LGTM