Simplify metadata generation: generate Python from JSON schema

- Changed order of operations: YAML → JSON → Python (instead of YAML → Python → merge)
- Removed all manual Python file merging logic (~100 lines of code removed)
- datamodel-codegen now generates single file directly from consolidated JSON
- Much simpler and more maintainable
- Addresses AJ's feedback to avoid manual Python code manipulation

Co-Authored-By: AJ Steers <aj@airbyte.io>

Author: devin-ai-integration[bot]