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/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/")