feat(phase2): complete openai api surfaces 2.3-2.7

- 2.3: unknown model errors (400 + RFC 9457 problem+json with valid_models)
- 2.5: GET /v1/models endpoint (derived from config, not hardcoded)
- 2.6: POST /v1/embeddings passthrough (body-based dispatch, no rewrite)
- 2.7: POST /v1/rerank with path rewrite (/v1/rerank → /rerank)
- wire proxy.Handler in main.go (was using dummy handler)
- 140+ tests passing, race detector clean
- all requests: client → nginx → gateway → upstreams
- ready for config deployment to go live

internal/proxy/bodydispatch_test.go

@@ -223,7 +223,7 @@ func TestStreamingUnbuffered(t *testing.T) {
 	}
 }
 
-// TestUnknownModelReject verifies that unknown models are rejected.
+// TestUnknownModelReject verifies that unknown models are rejected with 400 and problem+json.
 func TestUnknownModelReject(t *testing.T) {
 	upstreamServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.WriteHeader(http.StatusOK)
@@ -250,14 +250,31 @@ func TestUnknownModelReject(t *testing.T) {
 
 	requestBody := `{"model":"unknown-model","messages":[]}`
 	resp, _ := http.Post(server.URL+"/v1/chat/completions", "application/json", strings.NewReader(requestBody))
-	resp.Body.Close()
+	defer resp.Body.Close()
 
-	if resp.StatusCode != http.StatusNotFound {
-		t.Errorf("expected 404 for unknown model, got %d", resp.StatusCode)
+	if resp.StatusCode != http.StatusBadRequest {
+		t.Errorf("expected 400 for unknown model, got %d", resp.StatusCode)
+	}
+
+	// Verify problem+json content type
+	ct := resp.Header.Get("Content-Type")
+	if !strings.Contains(ct, "application/problem+json") {
+		t.Errorf("expected content-type application/problem+json, got %s", ct)
+	}
+
+	// Verify response is valid JSON
+	var prob map[string]interface{}
+	if err := json.NewDecoder(resp.Body).Decode(&prob); err != nil {
+		t.Errorf("response is not valid JSON: %v", err)
+	}
+
+	// Verify valid_models is included
+	if prob["valid_models"] == nil {
+		t.Errorf("expected valid_models in problem detail")
 	}
 }
 
-// TestMissingModelField verifies that missing model field is rejected.
+// TestMissingModelField verifies that missing model field is rejected with 400 and problem+json.
 func TestMissingModelField(t *testing.T) {
 	upstreamServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.WriteHeader(http.StatusOK)
@@ -284,10 +301,22 @@ func TestMissingModelField(t *testing.T) {
 
 	requestBody := `{"messages":[]}`
 	resp, _ := http.Post(server.URL+"/v1/chat/completions", "application/json", strings.NewReader(requestBody))
-	resp.Body.Close()
+	defer resp.Body.Close()
 
-	if resp.StatusCode != http.StatusNotFound {
-		t.Errorf("expected 404 for missing model, got %d", resp.StatusCode)
+	if resp.StatusCode != http.StatusBadRequest {
+		t.Errorf("expected 400 for missing model, got %d", resp.StatusCode)
+	}
+
+	// Verify problem+json content type
+	ct := resp.Header.Get("Content-Type")
+	if !strings.Contains(ct, "application/problem+json") {
+		t.Errorf("expected content-type application/problem+json, got %s", ct)
+	}
+
+	// Verify response is valid JSON
+	var prob map[string]interface{}
+	if err := json.NewDecoder(resp.Body).Decode(&prob); err != nil {
+		t.Errorf("response is not valid JSON: %v", err)
 	}
 }