fix(model): preserve custom endpoint credentials and accept cloud models not in /v1/models

When switching models on a custom endpoint (ollama-launch):
- Same-provider switches no longer re-resolve credentials (fixes base_url
  being lost for 'custom' provider on subsequent switches)
- Named providers (ollama-launch) are resolved via user_providers so
  switch_model can find their base_url from config
- Models not in the /v1/models probe but present in the user's saved
  provider config are accepted with a warning instead of rejected
- CLI /model and TUI /model both pass user_providers/custom_providers
  to switch_model so the config model list is available for validation

Closes #15088

tui_gateway/server.py

@@ -750,6 +750,18 @@ def _apply_model_switch(sid: str, session: dict, raw_input: str) -> dict:
         current_base_url = str(runtime.get("base_url", "") or "")
         current_api_key = str(runtime.get("api_key", "") or "")
 
+    # Load user-defined providers so switch_model can resolve named custom
+    # endpoints (e.g. "ollama-launch") and validate against saved model lists.
+    user_provs = None
+    custom_provs = None
+    try:
+        from hermes_cli.config import get_compatible_custom_providers, load_config
+        cfg = load_config()
+        user_provs = [{"provider": k, **v} for k, v in (cfg.get("providers") or {}).items()]
+        custom_provs = get_compatible_custom_providers(cfg)
+    except Exception:
+        pass
+
     result = switch_model(
         raw_input=model_input,
         current_provider=current_provider,
@@ -758,6 +770,8 @@ def _apply_model_switch(sid: str, session: dict, raw_input: str) -> dict:
         current_api_key=current_api_key,
         is_global=persist_global,
         explicit_provider=explicit_provider,
+        user_providers=user_provs,
+        custom_providers=custom_provs,
     )
     if not result.success:
         raise ValueError(result.error_message or "model switch failed")