fix: 恢复 thinking block 处理逻辑

- 修复合并冲突导致的逻辑错误
- Gemini 模型使用 dummy signature
- Claude 模型跳过无 signature 的 thinking block
- 删除未使用的 isValidThoughtSignature 函数

backend/internal/pkg/antigravity/request_transformer.go

@@ -172,31 +172,6 @@ func buildContents(messages []ClaudeMessage, toolIDToName map[string]string, isT
 // 参考: https://ai.google.dev/gemini-api/docs/thought-signatures
 const dummyThoughtSignature = "skip_thought_signature_validator"
 
-// isValidThoughtSignature 验证 thought signature 是否有效
-// Claude API 要求 signature 必须是 base64 编码的字符串，长度至少 32 字节
-func isValidThoughtSignature(signature string) bool {
-	// 空字符串无效
-	if signature == "" {
-		return false
-	}
-
-	// signature 应该是 base64 编码，长度至少 40 个字符（约 30 字节）
-	// 参考 Claude API 文档和实际观察到的有效 signature
-	if len(signature) < 40 {
-		return false
-	}
-
-	// 检查是否是有效的 base64 字符
-	// base64 字符集: A-Z, a-z, 0-9, +, /, =
-	for _, c := range signature {
-		if (c < 'A' || c > 'Z') && (c < 'a' || c > 'z') &&
-			(c < '0' || c > '9') && c != '+' && c != '/' && c != '=' {
-			return false
-		}
-	}
-
-	return true
-}
 // buildParts 构建消息的 parts
 // allowDummyThought: 只有 Gemini 模型支持 dummy thought signature
 func buildParts(content json.RawMessage, toolIDToName map[string]string, allowDummyThought bool) ([]GeminiPart, error) {
@@ -225,17 +200,22 @@ func buildParts(content json.RawMessage, toolIDToName map[string]string, allowDu
 			}
 
 		case "thinking":
-			// Claude 模型：仅在提供有效 signature 时保留 thinking block；否则跳过以避免上游校验失败。
-			signature := strings.TrimSpace(block.Signature)
-			if signature == "" || signature == dummyThoughtSignature {
-				log.Printf("[Warning] Skipping thinking block for Claude model (missing or dummy signature)")
-				continue
+			part := GeminiPart{
+				Text:    block.Thinking,
+				Thought: true,
 			}
-			parts = append(parts, GeminiPart{
-				Text:             block.Thinking,
-				Thought:          true,
-				ThoughtSignature: signature,
-			})
+			// 保留原有 signature（Claude 模型需要有效的 signature）
+			if block.Signature != "" {
+				part.ThoughtSignature = block.Signature
+			} else if !allowDummyThought {
+				// Claude 模型需要有效 signature，跳过无 signature 的 thinking block
+				log.Printf("Warning: skipping thinking block without signature for Claude model")
+				continue
+			} else {
+				// Gemini 模型使用 dummy signature
+				part.ThoughtSignature = dummyThoughtSignature
+			}
+			parts = append(parts, part)
 
 		case "image":
 			if block.Source != nil && block.Source.Type == "base64" {