- Restore deepseek-chat-v3-0324 and deepseek-reasoner with '(legacy)' labels;
these are deprecated 2026-07-24 but still live until then
- Fix zai (Z.AI/GLM) default_base_url: use /api/paas/v4 instead of /api/coding/paas/v4;
the coding plan path is for the glmcode custom provider, not the general API
- Update test assertions to match
- Test custom_providers entries (glmcode, deepseek) appear in get_providers()
- Test env var reference detection (${VAR_NAME} pattern)
- Test bare API key, missing key, empty/malformed entries
- Assert DeepSeek V4 models present, V3 deprecated models removed
- Assert GLM model series in _PROVIDER_MODELS and onboarding setup
