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/proxy/bodydispatch_test.go

@@ -381,3 +381,66 @@ func TestBodySizeCappedDispatch(t *testing.T) {
 		t.Errorf("expected 200 for reasonable body, got %d", resp.StatusCode)
 	}
 }
+
+// TestUpstreamModelRewrite verifies that the model field is rewritten when upstreamModel is set.
+func TestUpstreamModelRewrite(t *testing.T) {
+	var receivedModel string
+
+	upstreamServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		body, _ := io.ReadAll(r.Body)
+		var payload map[string]interface{}
+		json.Unmarshal(body, &payload)
+		receivedModel = payload["model"].(string)
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusOK)
+		fmt.Fprint(w, `{}`)
+	}))
+	defer upstreamServer.Close()
+
+	upstreamAddr := strings.TrimPrefix(upstreamServer.URL, "http://")
+
+	cfg := &config.Config{
+		Models: map[string]*config.ModelUpstream{
+			// Client sends "ornith:35b", upstream expects "qwen2.5:72b-instruct"
+			"ornith:35b": {
+				Name:          "ornith:35b",
+				Address:       upstreamAddr,
+				UpstreamModel: "qwen2.5:72b-instruct",
+			},
+			// No rewrite - upstream model same as client model
+			"reasoning": {
+				Name:    "reasoning",
+				Address: upstreamAddr,
+			},
+		},
+		Routes: make(map[string]*config.Route),
+	}
+
+	handler := New(cfg)
+	defer handler.Close()
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	// Test 1: Model should be rewritten
+	requestBody := `{"model":"ornith:35b","messages":[{"role":"user","content":"hi"}]}`
+	resp, err := http.Post(server.URL+"/v1/chat/completions", "application/json", strings.NewReader(requestBody))
+	if err != nil {
+		t.Fatalf("request failed: %v", err)
+	}
+	resp.Body.Close()
+
+	if receivedModel != "qwen2.5:72b-instruct" {
+		t.Errorf("expected upstream to receive model 'qwen2.5:72b-instruct', got '%s'", receivedModel)
+	}
+
+	// Test 2: No rewrite when upstreamModel is empty
+	receivedModel = ""
+	requestBody = `{"model":"reasoning","messages":[{"role":"user","content":"hi"}]}`
+	resp, _ = http.Post(server.URL+"/v1/chat/completions", "application/json", strings.NewReader(requestBody))
+	resp.Body.Close()
+
+	if receivedModel != "reasoning" {
+		t.Errorf("expected upstream to receive model 'reasoning', got '%s'", receivedModel)
+	}
+}

internal/proxy/router.go

@@ -122,6 +122,20 @@ func (h *Handler) routeByModel(r *http.Request, path string) (*Route, error) {
 		}
 	}
 
+	// If upstream expects a different model name, rewrite the body
+	if modelUpstream.UpstreamModel != "" && modelUpstream.UpstreamModel != modelName {
+		payload["model"] = modelUpstream.UpstreamModel
+		newBody, err := json.Marshal(payload)
+		if err != nil {
+			return nil, &modelValidationError{
+				Kind:    "invalid_request",
+				Message: fmt.Sprintf("failed to rewrite model name: %v", err),
+			}
+		}
+		r.Body = io.NopCloser(bytes.NewReader(newBody))
+		r.ContentLength = int64(len(newBody))
+	}
+
 	// Determine the upstream path based on the request path
 	upstreamPath := path
 	if path == "/v1/rerank" {