Add model generation from OpenAPI schemas

[xiaoland]

Summary

• replace Python 3.12-only type alias and generic syntax with Python 3.11 compatible annotations
• correct malformed f-strings and PostgREST query string assembly
• add missing runtime typing imports so OpenAPI generator tests can import the package

Testing

• PYTHONPATH=. pytest tests/test_scheme/test_openapi_generator.py -k openapi -q

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Codex Task