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/pkg/antigravity/request_transformer_test.go

@@ -15,15 +15,15 @@ func TestBuildParts_ThinkingBlockWithoutSignature(t *testing.T) {
 		description   string
 	}{
 		{
-			name: "Claude model - skip thinking block without signature",
+			name: "Claude model - drop thinking without signature",
 			content: `[
 				{"type": "text", "text": "Hello"},
 				{"type": "thinking", "thinking": "Let me think...", "signature": ""},
 				{"type": "text", "text": "World"}
 			]`,
 			thoughtMode:   thoughtSignatureModePreserve,
-			expectedParts: 2, // 只有两个text block
-			description:   "Claude模型应该跳过无signature的thinking block",
+			expectedParts: 2, // thinking 内容被丢弃
+			description:   "Claude模型应丢弃无signature的thinking block内容",
 		},
 		{
 			name: "Claude model - preserve thinking block with signature",