feat: add upstreamModel config for model name mapping

When the upstream LLM server expects a different model name than what
clients send, the gateway now rewrites the 'model' field in the request
body before forwarding.

Config example:
  models:
  - name: "ornith:35b"           # client-facing name
    address: "ornith-predictor:80"
    upstreamModel: "qwen2.5:72b"  # what upstream expects

Fixes 400 'model is required' errors when upstream model names differ.

internal/config/loader.go

@@ -37,10 +37,11 @@ type rawRoute struct {
 
 // rawModel represents a single model entry in the YAML configuration.
 type rawModel struct {
-	Name         string `yaml:"name"`
-	Address      string `yaml:"address"`
-	Path         string `yaml:"path"`
-	AuthRequired *bool  `yaml:"authRequired"`
+	Name          string `yaml:"name"`
+	Address       string `yaml:"address"`
+	Path          string `yaml:"path"`
+	UpstreamModel string `yaml:"upstreamModel"`
+	AuthRequired  *bool  `yaml:"authRequired"`
 }
 
 // rawAdapter represents a service adapter in the YAML configuration.
@@ -132,10 +133,11 @@ func LoadRoutesAndModelsFromFile(path string) (map[string]*Route, map[string]*Mo
 			authRequired = *rawModel.AuthRequired
 		}
 		models[rawModel.Name] = &ModelUpstream{
-			Name:         rawModel.Name,
-			Address:      rawModel.Address,
-			Path:         rawModel.Path,
-			AuthRequired: authRequired,
+			Name:          rawModel.Name,
+			Address:       rawModel.Address,
+			Path:          rawModel.Path,
+			UpstreamModel: rawModel.UpstreamModel,
+			AuthRequired:  authRequired,
 		}
 	}