fix: generate valid code when a path parameter with a default precedes one without - #1495

Summary

Generating a client for an endpoint whose path declares a parameter with a default before a parameter without one (for example /test/{has_default}/{no_default}, where has_default has a schema default) produced invalid Python:

def _get_kwargs(
    has_default: str = "default_value",
    no_default: int,
) -> dict[str, Any]:

Python rejects this with SyntaxError: Parameter without a default cannot follow a parameter with a default, so generation failed entirely.

Changes

• Endpoint.sort_parameters now stable-sorts path parameters after ordering them by their position in the path, moving parameters that declare a default after those that do not. Path order is preserved within each group, so endpoints that already generated valid code are unchanged, and all internal calls pass path parameters by keyword.
• Added a unit test covering the reordering, including preservation of path order within the with-default and without-default groups.
• Added functional tests that generate a client from the failing shape of spec, verify the default is used when the parameter is omitted, and verify an explicit value overrides it.

Tests

• pytest tests/test_parser/test_openapi.py end_to_end_tests/functional_tests/generated_code_execution/test_path_parameters.py — 33 passed.
• pytest tests --basetemp=tests/tmp — 322 passed, 4 skipped.
• pytest end_to_end_tests/test_end_to_end.py end_to_end_tests/functional_tests --basetemp=tests/tmp — 136 passed; the 9 snapshot failures in test_end_to_end.py occur identically on unmodified main and are unaffected by this change.
• Manual verification: generating a client from the spec in Generates SyntaxError if a path parameter without a default value follows another that has a default value #1281 failed before this change and succeeds after it, producing no_default: int, has_default: str = "default_value".
• ruff check and ruff format --check pass on the changed files; mypy openapi_python_client/parser/openapi.py reports no issues.

Fixes #1281