Cover lower-case API key headers

[kriptoburak]

Description

Adds a client-rendering regression test for an OpenAPI 3.1 shape with a lower-case header apiKey scheme. The test verifies the default base URL, constructor parameter, auth middleware target, and generated method binding.

Fixes: N/A

Type of change

• Tests only: coverage for existing auth rendering behavior

Checklist

• uv run ruff check tests/test_clients_extra.py
• uv run ruff format --check tests/test_clients_extra.py
• I have conducted a self-review of my own code
• Documentation is not required for this coverage-only change
• My changes do not generate any new warnings
• I have added tests to validate the covered behavior
• uv run mypy
• The OpenAPI fixture illustrates the covered behavior

Tests

• uv run pytest tests/test_clients_extra.py tests/test_render_auth_errors.py
• uv run ruff check tests/test_clients_extra.py
• uv run ruff format --check tests/test_clients_extra.py
• uv run mypy