feat: honor OpenAI-compatible base URLs

[debgotwired]

Summary

• honor configured base_url for OpenAI-compatible endpoints
• preserve Ollama localhost fallback when no explicit compatible host is configured
• add deterministic OpenAI handler tests for OpenAI, Ollama, OpenRouter-style, LM Studio-style, and custom Ollama hosts

Testing

• python3 -m py_compile readmeai/models/openai.py tests/models/test_openai.py
• git diff --check
• poetry run pytest -o addopts='' tests/models/test_openai.py -n 0