feat(providers): add GMI Cloud as a first-class API-key provider (#11955)

Add GMI Cloud (api.gmi-serving.com) as a full first-class API-key provider
with built-in auth, aliases, model catalog, CLI entry points, auxiliary client
routing, context length resolution, doctor checks, env var tracking, and docs.

- auth.py: ProviderConfig for 'gmi' (api_key, GMI_API_KEY / GMI_BASE_URL)
- providers.py: HermesOverlay with extra_env_vars for models.dev detection
- models.py: curated slash-form model catalog; live /v1/models fetch
- main.py: 'gmi' in _named_custom_provider_map and --provider choices
- model_metadata.py: _URL_TO_PROVIDER, _PROVIDER_PREFIXES, dedicated
  context-length probe block (GMI's /models has authoritative data)
- auxiliary_client.py: alias entries; _compat_model fix for slash-form
  models on cached aggregator-style clients; gmi aux default model
- doctor.py: GMI in provider connectivity checks
- config.py: GMI_API_KEY / GMI_BASE_URL in OPTIONAL_ENV_VARS
- conftest.py: explicit GMI_BASE_URL clearing (not caught by _API_KEY suffix)
- docs: providers.md, environment-variables.md, fallback-providers.md,
  configuration.md, quickstart.md (expands provider table)

Co-authored-by: Isaac Huang <isaachuang@Isaacs-MacBook-Pro.local>

website/docs/user-guide/features/fallback-providers.md

@@ -59,6 +59,7 @@ Both `provider` and `model` are **required**. If either is missing, the fallback
 | Kilo Code | `kilocode` | `KILOCODE_API_KEY` |
 | Xiaomi MiMo | `xiaomi` | `XIAOMI_API_KEY` |
 | Arcee AI | `arcee` | `ARCEEAI_API_KEY` |
+| GMI Cloud | `gmi` | `GMI_API_KEY` |
 | Alibaba / DashScope | `alibaba` | `DASHSCOPE_API_KEY` |
 | Hugging Face | `huggingface` | `HF_TOKEN` |
 | Custom endpoint | `custom` | `base_url` + `key_env` (see below) |