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/main.py

@@ -2945,7 +2945,7 @@ def _model_flow_named_custom(config, provider_info):
 
 # Curated model lists for direct API-key providers — single source in models.py
 from hermes_cli.models import _PROVIDER_MODELS
-from hermes_cli.provider_contracts import (
+from hermes_cli.volcengine_byteplus import (
     base_url_for_provider_model,
     provider_models,
 )