Fix/custom local

[Verdenroz]

Summary

• Adds CUSTOM = "custom" to the Provider enum so that the custom local endpoint registered via base_url= is a valid enum member.
• Registers Provider.CUSTOM: _openai_format in _FORMAT_BUILDERS so that structured outputs (response_model=) work against any OpenAI-compatible local server (llama.cpp, llama-swap, Ollama, etc.).
• Fixes the providers property, which iterates all registered providers and calls Provider(name) — this would crash whenever a custom endpoint was configured.

Breaking Changes

None

Test Plan

• Chimeric(base_url="http://127.0.0.1:12434/v1").list_models() returns models without error
• client.generate(model=..., messages=..., response_model=MyModel) parses structured output from a local provider

Checklist

• Tests pass (nox -s tests)
• Coverage ≥ 80%
• Docs updated (if public API changed)
• No hardcoded secrets or credentials

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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