feat(antigravity): 增强网关功能和 thinking 块处理

主要改进：
- 优化 thinking blocks 过滤策略，支持 Auto 模式降级
- 将无效 thinking block 内容转为普通 text
- 保留单个空白 text block，不过滤
- 重构配额刷新机制，统一与 Claude 一致
- 支持 cachedContentTokenCount 映射到 cache_read_input_tokens
- Haiku 模型映射到 Sonnet
- 添加 /antigravity/v1/models 端点支持
- countTokens 端点直接返回空值

backend/internal/service/antigravity_model_mapping_test.go

@@ -104,28 +104,28 @@ func TestAntigravityGatewayService_GetMappedModel(t *testing.T) {
 			expected:       "claude-opus-4-5-thinking",
 		},
 		{
-			name:           "系统映射 - claude-haiku-4 → gemini-3-flash",
+			name:           "系统映射 - claude-haiku-4 → claude-sonnet-4-5",
 			requestedModel: "claude-haiku-4",
 			accountMapping: nil,
-			expected:       "gemini-3-flash",
+			expected:       "claude-sonnet-4-5",
 		},
 		{
-			name:           "系统映射 - claude-haiku-4-5 → gemini-3-flash",
+			name:           "系统映射 - claude-haiku-4-5 → claude-sonnet-4-5",
 			requestedModel: "claude-haiku-4-5",
 			accountMapping: nil,
-			expected:       "gemini-3-flash",
+			expected:       "claude-sonnet-4-5",
 		},
 		{
-			name:           "系统映射 - claude-3-haiku-20240307 → gemini-3-flash",
+			name:           "系统映射 - claude-3-haiku-20240307 → claude-sonnet-4-5",
 			requestedModel: "claude-3-haiku-20240307",
 			accountMapping: nil,
-			expected:       "gemini-3-flash",
+			expected:       "claude-sonnet-4-5",
 		},
 		{
-			name:           "系统映射 - claude-haiku-4-5-20251001 → gemini-3-flash",
+			name:           "系统映射 - claude-haiku-4-5-20251001 → claude-sonnet-4-5",
 			requestedModel: "claude-haiku-4-5-20251001",
 			accountMapping: nil,
-			expected:       "gemini-3-flash",
+			expected:       "claude-sonnet-4-5",
 		},
 		{
 			name:           "系统映射 - claude-sonnet-4-5-20250929",