refactor: extract resolve_model_provider helper, fix cross-provider routing

Replace duplicated inline provider resolution in routes.py and streaming.py
with a shared resolve_model_provider() helper in config.py.

Improvements over original:
- If model ID has a prefix matching any known direct-API provider
  (not just the config provider), strip it and route correctly.
  This handles edge cases like localStorage restoring a model from
  a different provider group.
- Single source of truth for the resolution logic.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/streaming.py

@@ -13,7 +13,7 @@ from pathlib import Path
 from api.config import (
     STREAMS, STREAMS_LOCK, CANCEL_FLAGS, CLI_TOOLSETS,
     _get_session_agent_lock, _set_thread_env, _clear_thread_env,
-    cfg as _hermes_cfg,
+    resolve_model_provider,
 )
 
 # Lazy import to avoid circular deps -- hermes-agent is on sys.path via api/config.py
@@ -100,18 +100,10 @@ def _run_agent_streaming(session_id, msg_text, model, workspace, stream_id, atta
 
             if AIAgent is None:
                 raise ImportError("AIAgent not available -- check that hermes-agent is on sys.path")
-            # Resolve provider from config so agent routes to the right API
-            _provider = None
-            model_cfg = _hermes_cfg.get('model', {})
-            if isinstance(model_cfg, dict):
-                _provider = model_cfg.get('provider')
-            # If model has provider/ prefix matching config provider, strip it
-            # so AIAgent doesn't misroute to OpenRouter
-            if _provider and '/' in model and model.startswith(_provider + '/'):
-                model = model.split('/', 1)[1]
+            resolved_model, resolved_provider = resolve_model_provider(model)
             agent = AIAgent(
-                model=model,
-                provider=_provider,
+                model=resolved_model,
+                provider=resolved_provider,
                 platform='cli',
                 quiet_mode=True,
                 enabled_toolsets=CLI_TOOLSETS,