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

hermes_cli/runtime_provider.py

@@ -643,7 +643,7 @@ def _resolve_explicit_runtime(
 
         base_url = explicit_base_url
         if not base_url:
-            if provider in ("kimi-coding", "kimi-coding-cn"):
+            if provider in ("kimi-coding", "kimi-coding-cn", "volcengine", "byteplus"):
                 creds = resolve_api_key_provider_credentials(provider)
                 base_url = creds.get("base_url", "").rstrip("/")
             else: