feat(providers): add Volcengine and BytePlus support

Based on PR #8952 by @Maaannnn.

Adds Volcengine and BytePlus as first-class providers, each with standard
and Coding Plan model catalogs. The model prefix (volcengine/ vs
volcengine-coding-plan/) determines the runtime base URL automatically.

- New hermes_cli/provider_contracts.py centralises all constants
- ProviderConfig entries in auth.py with api_key auth
- Model catalogs, aliases, and provider ordering in models.py/providers.py
- Auxiliary client entries and context window resolution
- gateway /provider command detects known Volcengine/BytePlus endpoints
- Comprehensive tests and docs update

tests/agent/test_model_metadata.py

@@ -222,6 +222,22 @@ class TestGetModelContextLength:
         mock_fetch.return_value = {}
         assert get_model_context_length("unknown/never-heard-of-this") == CONTEXT_PROBE_TIERS[0]
 
+    @patch("agent.model_metadata.fetch_model_metadata")
+    def test_volcengine_contract_model_uses_contract_context_length(self, mock_fetch):
+        mock_fetch.return_value = {}
+        assert get_model_context_length(
+            "volcengine/doubao-seed-2-0-pro-260215",
+            provider="volcengine",
+        ) == 256000
+
+    @patch("agent.model_metadata.fetch_model_metadata")
+    def test_byteplus_contract_model_infers_provider_from_url(self, mock_fetch):
+        mock_fetch.return_value = {}
+        assert get_model_context_length(
+            "byteplus-coding-plan/kimi-k2.5",
+            base_url="https://ark.ap-southeast.bytepluses.com/api/coding/v3",
+        ) == 256000
+
     @patch("agent.model_metadata.fetch_model_metadata")
     def test_partial_match_in_defaults(self, mock_fetch):
         mock_fetch.return_value = {}