Enhance linting workflow to better handle warnings and errors.

[dshevtsov]

Added checks for fatal errors and improved output messages for clarity. Now captures unexpected errors during linting and updates the summary accordingly.

This pull request updates the linting workflow in .github/workflows/validate-pr.yml to improve detection and reporting of warnings and errors. The changes make the workflow more robust by distinguishing between warnings, fatal errors, and unexpected failures, ensuring clearer feedback in GitHub Actions.

Linting workflow improvements:

• Added detection for "❌" markers in lint-output.txt to better identify issues during linting.
• Updated logic to treat non-zero exit codes as fatal errors only if no warning markers are found, providing more accurate error reporting.
• Enhanced error handling: unexpected linting failures now display the output in the GitHub summary and terminate the workflow with an error.
• Improved status file writing: has-warnings.txt is set to "false" for fatal errors and "true" for warnings, clarifying downstream workflow behavior.
• Ensured all workflow branches properly clean up temporary files and update summary messages for better maintainability.