fix: Add non-JSON and empty error bodies handling

[ewuerger]

Solve #84

Explanation why fix is done with regex in build_client.py (post-generation-fix):

Summary

• Regex correctness: confirmed - produces identical output to the manual patch
• Path correctness: confirmed - output_path / "api" resolves to api, the directory with all 220 files
• Why regex is the right call here: you're correct that the template approach was ruled out. The Errors.from_dict(response.json()) line is assembled across three template files (endpoint_module.py.jinja → endpoint_macros.py.jinja → model_property.py.jinja → property_macros.py.jinja). The only hook point is parse_response in endpoint_macros.py.jinja, but overriding that requires duplicating the full upstream macro which includes unrelated body-parsing logic, and would silently break on any generator update. Post-processing in build_client.py is the established pattern there (it already does regex manipulation on the OpenAPI spec before generation) and the target string response_NNN = Errors.from_dict(response.json()) is 100% generator-stable - that exact form appears nowhere else and will always be produced by model_property.py.jinja.