fix: follow-up for salvaged PR #8952

- Rename provider_contracts.py -> volcengine_byteplus.py for explicitness
- Consolidate duplicate host-to-provider mappings: provider_for_base_url()
  now uses the canonical _URL_TO_PROVIDER from model_metadata.py instead of
  maintaining a separate 20-entry dict
- Add volcengine/byteplus to runtime_provider.py model-dependent base URL
  resolution (kimi-style special case) so manually-edited configs resolve
  the coding-plan base URL correctly
- Remove volcengine/byteplus from _API_KEY_PROVIDER_AUX_MODELS — the
  main-model-first design in _resolve_auto() handles these providers
  already; entries were dead code in the normal flow
- Add VOLCENGINE_API_KEY and BYTEPLUS_API_KEY to OPTIONAL_ENV_VARS in
  config.py so they appear in hermes setup
- Update docs: environment-variables.md, fallback-providers.md,
  configuration.md

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

@@ -58,6 +58,8 @@ Both `provider` and `model` are **required**. If either is missing, the fallback
 | OpenCode Go | `opencode-go` | `OPENCODE_GO_API_KEY` |
 | Kilo Code | `kilocode` | `KILOCODE_API_KEY` |
 | Xiaomi MiMo | `xiaomi` | `XIAOMI_API_KEY` |
+| Volcengine | `volcengine` | `VOLCENGINE_API_KEY` |
+| BytePlus | `byteplus` | `BYTEPLUS_API_KEY` |
 | Arcee AI | `arcee` | `ARCEEAI_API_KEY` |
 | Alibaba / DashScope | `alibaba` | `DASHSCOPE_API_KEY` |
 | Hugging Face | `huggingface` | `HF_TOKEN` |