fix: resolve critical SDK bugs in generate and test files

[RAVEN1106]

Summary

This PR fixes critical bugs in the Genstack Python SDK that were causing failures in core functionality.

Changes Made

genstack/genstack.py

• Fixed error handling in the generate function
• Resolved issues with API response parsing
• Improved stability and reliability of core SDK methods

tests/test_generate.py

• Updated test cases to reflect corrected behavior
• Added proper assertions for fixed functionality
• Ensured all critical code paths are tested

Testing

All tests have been updated and pass with the fixes applied.

Impact

These fixes resolve critical SDK bugs that affected users relying on core generate functionality.