fix(gemini): 更新 Gemini 模型列表配置

- 移除已弃用的 1.5 系列模型
- 调整模型优先级顺序（2.0 Flash > 2.5 Flash > 2.5 Pro > 3.0 Preview）
- 同步前后端模型配置
- 更新相关测试用例和默认模型选择逻辑

backend/internal/service/antigravity_model_mapping_test.go

@@ -30,7 +30,7 @@ func TestIsAntigravityModelSupported(t *testing.T) {
 		{"可映射 - claude-3-haiku-20240307", "claude-3-haiku-20240307", true},
 
 		// Gemini 前缀透传
-		{"Gemini前缀 - gemini-1.5-pro", "gemini-1.5-pro", true},
+		{"Gemini前缀 - gemini-2.5-pro", "gemini-2.5-pro", true},
 		{"Gemini前缀 - gemini-unknown-model", "gemini-unknown-model", true},
 		{"Gemini前缀 - gemini-future-version", "gemini-future-version", true},
 
@@ -142,10 +142,10 @@ func TestAntigravityGatewayService_GetMappedModel(t *testing.T) {
 			expected:       "gemini-2.5-flash",
 		},
 		{
-			name:           "Gemini透传 - gemini-1.5-pro",
-			requestedModel: "gemini-1.5-pro",
+			name:           "Gemini透传 - gemini-2.5-pro",
+			requestedModel: "gemini-2.5-pro",
 			accountMapping: nil,
-			expected:       "gemini-1.5-pro",
+			expected:       "gemini-2.5-pro",
 		},
 		{
 			name:           "Gemini透传 - gemini-future-model",

backend/internal/service/gemini_multiplatform_test.go

@@ -599,7 +599,7 @@ func TestGeminiMessagesCompatService_isModelSupportedByAccount(t *testing.T) {
 			name: "Gemini平台-有映射配置-只支持配置的模型",
 			account: &Account{
 				Platform:    PlatformGemini,
-				Credentials: map[string]any{"model_mapping": map[string]any{"gemini-1.5-pro": "x"}},
+				Credentials: map[string]any{"model_mapping": map[string]any{"gemini-2.5-pro": "x"}},
 			},
 			model:    "gemini-2.5-flash",
 			expected: false,

backend/internal/service/pricing_service.go

@@ -531,8 +531,8 @@ func (s *PricingService) buildModelLookupCandidates(modelLower string) []string
 func normalizeModelNameForPricing(model string) string {
 	// Common Gemini/VertexAI forms:
 	// - models/gemini-2.0-flash-exp
-	// - publishers/google/models/gemini-1.5-pro
-	// - projects/.../locations/.../publishers/google/models/gemini-1.5-pro
+	// - publishers/google/models/gemini-2.5-pro
+	// - projects/.../locations/.../publishers/google/models/gemini-2.5-pro
 	model = strings.TrimSpace(model)
 	model = strings.TrimLeft(model, "/")
 	model = strings.TrimPrefix(model, "models/")