Add z.ai provider support

[magix022]

Summary

• Add z.ai as a LiteLLM-backed API-key provider using the zai/ runtime prefix.
• Default the setup menu to glm-5.2 with curated GLM alternatives: glm-5.1, glm-4.7, glm-4.6, glm-4.5, and glm-4.5-flash.
• Document z.ai in provider docs and clarify that setup model menus are curated, not exhaustive.

Auth and secrets

• Uses ZAI_API_KEY by default, following Fractal's existing env/stored credential flow.
• No raw API keys or secrets are stored in config.

Model sources

• LiteLLM documents the Z.AI provider prefix as zai/ and ZAI_API_KEY.
• Z.AI docs list GLM-5.2 and GLM-5.1 in the current model overview; LiteLLM documents the requested GLM 4.x model IDs.

Tests

• .venv/bin/python -m pytest tests/test_providers.py tests/test_cli_config.py
• .venv/bin/python -m pytest

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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