Improve robustness of env parsing and API error handling

[Copilot]

Summary

This PR introduces targeted code quality improvements to make the action more robust and fault-tolerant.

Changes

• Hardened environment variable handling in main.py:
  • Added validated progress-bar length parsing with safe fallback
  • Clamped progress values to a valid 0-100 range
  • Reworked progress-bar assembly for clearer behavior
  • Added boolean parsing for INPUT_SHOW_EP_INFO to correctly handle values like false
• Improved resilience in readme_level.py:
  • Added explicit handling for missing GitHub token
  • Added request exception handling for GitHub API calls
  • Added non-200 response handling
  • Added malformed API payload handling
  • Prevented crashes by handling missing user data in EP calculation
• Extended tests:
  • Added tests for missing token and failed API response paths
  • Added test for EP calculation when user data is unavailable
  • Added test to ensure EP info is hidden when INPUT_SHOW_EP_INFO=false

Validation

• python -m unittest ✅
• pylint $(git ls-files '*.py') ⚠️ (existing unrelated lint issue remains in graphql_query.py line length)
• Parallel validation:
  • Code Review ✅ (no comments after fixes)
  • CodeQL Security Scan ✅ (0 alerts)