fix(custom-providers): propagate model field from config to runtime so API receives the correct model name

Fixes #7828

When a custom_providers entry carries a `model` field, that value was
silently dropped by `_get_named_custom_provider` and
`_resolve_named_custom_runtime`.  Callers received a runtime dict with
`base_url`, `api_key`, and `api_mode` — but no `model`.

As a result, `hermes chat --model <provider-name>` sent the *provider
name* (e.g. "my-dashscope-provider") as the model string to the API
instead of the configured model (e.g. "qwen3.6-plus"), producing:

    Error code: 400 - {'error': {'message': 'Model Not Exist'}}

Setting the provider as the *default* model in config.yaml worked
because that path writes `model.default` and the agent reads it back
directly, bypassing the broken runtime resolution path.

Changes:

1. hermes_cli/runtime_provider.py — _get_named_custom_provider()
   Reads `entry.get("model")` and includes it in the result dict so
   the value is available to callers.

2. hermes_cli/runtime_provider.py — _resolve_named_custom_runtime()
   Propagates `custom_provider["model"]` into the returned runtime dict.

3. cli.py — _ensure_runtime_credentials()
   After resolving runtime, if `runtime["model"]` is set, assign it to
   `self.model` so the AIAgent is initialised with the correct model
   name rather than the provider name the user typed on the CLI.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cli.py

@@ -2710,6 +2710,15 @@ class HermesCLI:
         self.api_key = api_key
         self.base_url = base_url
 
+        # When a custom_provider entry carries an explicit `model` field,
+        # use it as the effective model name.  Without this, running
+        # `hermes chat --model <provider-name>` sends the provider name
+        # (e.g. "my-provider") as the model string to the API instead of
+        # the configured model (e.g. "qwen3.6-plus"), causing 400 errors.
+        runtime_model = runtime.get("model")
+        if runtime_model and isinstance(runtime_model, str):
+            self.model = runtime_model
+
         # Normalize model for the resolved provider (e.g. swap non-Codex
         # models when provider is openai-codex).  Fixes #651.
         model_changed = self._normalize_model_for_provider(resolved_provider)