Refactor wildcard out of CORS headers (#81)

Changes to CORS functionality: 

- `Access-Control-Allow-Origin: *` is set for all requests 
- for pre-flight OPTIONS requests
  - specify methods: `Access-Control-Allow-Methods: GET, POST, PUT, PATCH, DELETE, OPTIONS`
  - if the client sent `Access-Control-Request-Headers` then echo back the same value in `Access-Control-Allow-Headers`. If no `Access-Control-Request-Headers` were sent, then send back a default set
  - set `Access-Control-Max-Age: 86400` to that may improve performance 
- Add CORS tests to the proxy-manager

proxy/proxymanager_test.go

@@ -639,5 +639,95 @@ func TestProxyManager_UseModelName(t *testing.T) {
 			assert.Equal(t, upstreamModelName, response["model"])
 		})
 	}
-
+}
+
+func TestProxyManager_CORSOptionsHandler(t *testing.T) {
+	config := &Config{
+		HealthCheckTimeout: 15,
+		Models: map[string]ModelConfig{
+			"model1": getTestSimpleResponderConfig("model1"),
+		},
+		LogRequests: true,
+	}
+
+	tests := []struct {
+		name            string
+		method          string
+		requestHeaders  map[string]string
+		expectedStatus  int
+		expectedHeaders map[string]string
+	}{
+		{
+			name:           "OPTIONS with no headers",
+			method:         "OPTIONS",
+			expectedStatus: http.StatusNoContent,
+			expectedHeaders: map[string]string{
+				"Access-Control-Allow-Origin":  "*",
+				"Access-Control-Allow-Methods": "GET, POST, PUT, PATCH, DELETE, OPTIONS",
+				"Access-Control-Allow-Headers": "Content-Type, Authorization, Accept, X-Requested-With",
+			},
+		},
+		{
+			name:   "OPTIONS with specific headers",
+			method: "OPTIONS",
+			requestHeaders: map[string]string{
+				"Access-Control-Request-Headers": "X-Custom-Header, Some-Other-Header",
+			},
+			expectedStatus: http.StatusNoContent,
+			expectedHeaders: map[string]string{
+				"Access-Control-Allow-Origin":  "*",
+				"Access-Control-Allow-Methods": "GET, POST, PUT, PATCH, DELETE, OPTIONS",
+				"Access-Control-Allow-Headers": "X-Custom-Header, Some-Other-Header",
+			},
+		},
+		{
+			name:           "Non-OPTIONS request",
+			method:         "GET",
+			expectedStatus: http.StatusNotFound, // Since we don't have a GET route defined
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			proxy := New(config)
+			defer proxy.StopProcesses()
+
+			req := httptest.NewRequest(tt.method, "/v1/chat/completions", nil)
+			for k, v := range tt.requestHeaders {
+				req.Header.Set(k, v)
+			}
+
+			w := httptest.NewRecorder()
+			proxy.ginEngine.ServeHTTP(w, req)
+
+			assert.Equal(t, tt.expectedStatus, w.Code)
+
+			for header, expectedValue := range tt.expectedHeaders {
+				assert.Equal(t, expectedValue, w.Header().Get(header))
+			}
+		})
+	}
+}
+
+func TestProxyManager_CORSHeadersInRegularRequest(t *testing.T) {
+	config := &Config{
+		HealthCheckTimeout: 15,
+		Models: map[string]ModelConfig{
+			"model1": getTestSimpleResponderConfig("model1"),
+		},
+		LogRequests: true,
+	}
+
+	proxy := New(config)
+	defer proxy.StopProcesses()
+
+	// Test that CORS headers are present in regular POST requests
+	reqBody := `{"model":"model1"}`
+	req := httptest.NewRequest("POST", "/v1/chat/completions", bytes.NewBufferString(reqBody))
+	w := httptest.NewRecorder()
+
+	proxy.ginEngine.ServeHTTP(w, req)
+
+	assert.Equal(t, http.StatusOK, w.Code)
+	assert.Equal(t, "*", w.Header().Get("Access-Control-Allow-Origin"))
 }