refactor(convert): dynamically detect languages and versions instead …

[Arunodoy18]

Summary

Refactors ConvertVars to dynamically detect available languages and versions by scanning the source/ directory at runtime instead of relying on hardcoded lists.

Closes #2340.

Problem

ConvertVars previously used hardcoded LANGUAGE_CHOICES and VERSION_CHOICES.
This required manual updates whenever new YAML files were added and created a risk of inconsistencies between the script and the actual contents of source/.

Changes

• Scan the source/ directory for .yaml files during initialization.
• Extract version and language from filename patterns.
• Dynamically populate:
  • LANGUAGE_CHOICES
  • VERSION_CHOICES
• Preserve special values: "all" and "latest".
• Maintain existing CLI behavior (no breaking changes).

Benefits

• Eliminates manual maintenance when new files are added.
• Prevents mismatch between script configuration and available source files.
• Improves contributor discoverability.
• Reduces technical debt.

Notes

• Directory scanning occurs once during initialization.
• Malformed filenames are handled safely.
• No changes to conversion logic or output format.