fix: multi-provider model routing via @provider: hint (#138) (#146)

The previous fix (#142) prefixed non-default provider models with
'provider/model' which then hit the cross-provider guard and routed
to OpenRouter — worse than before for users without an OpenRouter key.

New approach: non-default provider models use '@provider:model' format
(e.g. @minimax:MiniMax-M2.7). resolve_model_provider() parses this
hint and returns (bare_model, provider, None). streaming.py and
routes.py then pass the resolved provider to
resolve_runtime_provider(requested=provider) which gets the correct
per-provider API key and base_url from hermes-agent.

This uses the agent's own credential resolution instead of reinventing
routing logic in the webui.

Fixes #138

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

api/routes.py

@@ -1061,7 +1061,7 @@ def _handle_chat_sync(handler, body):
             _api_key = None
             try:
                 from hermes_cli.runtime_provider import resolve_runtime_provider
-                _rt = resolve_runtime_provider()
+                _rt = resolve_runtime_provider(requested=_provider)
                 _api_key = _rt.get("api_key")
                 # Also use runtime provider/base_url if the webui config didn't resolve them
                 if not _provider: