fix: changelog comparison URL in release workflow

[vanducng]

Summary

• Fix changelog comparison URL that was comparing the same version against itself
• Now correctly compares against the previous release tag to show actual changes

Problem

The release workflow was generating changelog URLs like v1.0.0...v1.0.0 which shows no changes, instead of comparing against the previous release.

Solution

# Before
compare/v${{ steps.extract_version.outputs.version }}...v${{ steps.extract_version.outputs.version }}

# After  
compare/${{ steps.previous_tag.outputs.previous_tag }}...v${{ steps.extract_version.outputs.version }}

Added a step to get the previous release tag using git describe --tags --abbrev=0 HEAD~1.

Test plan

• Verify workflow runs without errors
• Check that changelog URLs in releases now show actual changes between versions

[ltawfik]

Thanks!