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/config.py

@@ -1281,6 +1281,20 @@ OPTIONAL_ENV_VARS = {
         "category": "provider",
         "advanced": True,
     },
+    "VOLCENGINE_API_KEY": {
+        "description": "Volcengine API key for Doubao / Seed models (standard + Coding Plan catalogs)",
+        "prompt": "Volcengine API Key",
+        "url": "https://www.volcengine.com/product/ark",
+        "password": True,
+        "category": "provider",
+    },
+    "BYTEPLUS_API_KEY": {
+        "description": "BytePlus API key for Seed / Dola models (standard + Coding Plan catalogs)",
+        "prompt": "BytePlus API Key",
+        "url": "https://www.byteplus.com/en/product/modelark",
+        "password": True,
+        "category": "provider",
+    },
     "AWS_REGION": {
         "description": "AWS region for Bedrock API calls (e.g. us-east-1, eu-central-1)",
         "prompt": "AWS Region",