revert(antigravity): 恢复 Claude 模型 thinking 功能

还原 b6d1e7a0 中错误禁用 Claude thinking 的逻辑：
- 移除 isThinkingEnabled 对 allowDummyThought 的依赖
- 移除非 Gemini 模型时清除 Thinking 配置的代码
- 恢复 buildParts 中 thinking block 的原始处理逻辑
- 移除不再使用的 isValidThoughtSignature 函数

backend/internal/pkg/antigravity/request_transformer.go

@@ -14,16 +14,13 @@ func TransformClaudeToGemini(claudeReq *ClaudeRequest, projectID, mappedModel st
 	// 用于存储 tool_use id -> name 映射
 	toolIDToName := make(map[string]string)
 
+	// 检测是否启用 thinking
+	isThinkingEnabled := claudeReq.Thinking != nil && claudeReq.Thinking.Type == "enabled"
+
 	// 只有 Gemini 模型支持 dummy thought workaround
 	// Claude 模型通过 Vertex/Google API 需要有效的 thought signatures
 	allowDummyThought := strings.HasPrefix(mappedModel, "gemini-")
 
-	// 检测是否启用 thinking
-	requestedThinkingEnabled := claudeReq.Thinking != nil && claudeReq.Thinking.Type == "enabled"
-	// 为避免 Claude 模型的 thought signature/消息块约束导致 400（上游要求 thinking 块开头等），
-	// 非 Gemini 模型默认不启用 thinking（除非未来支持完整签名链路）。
-	isThinkingEnabled := requestedThinkingEnabled && allowDummyThought
-
 	// 1. 构建 contents
 	contents, err := buildContents(claudeReq.Messages, toolIDToName, isThinkingEnabled, allowDummyThought)
 	if err != nil {
@@ -34,15 +31,7 @@ func TransformClaudeToGemini(claudeReq *ClaudeRequest, projectID, mappedModel st
 	systemInstruction := buildSystemInstruction(claudeReq.System, claudeReq.Model)
 
 	// 3. 构建 generationConfig
-	reqForGen := claudeReq
-	if requestedThinkingEnabled && !allowDummyThought {
-		log.Printf("[Warning] Disabling thinking for non-Gemini model in antigravity transform: model=%s", mappedModel)
-		// shallow copy to avoid mutating caller's request
-		clone := *claudeReq
-		clone.Thinking = nil
-		reqForGen = &clone
-	}
-	generationConfig := buildGenerationConfig(reqForGen)
+	generationConfig := buildGenerationConfig(claudeReq)
 
 	// 4. 构建 tools
 	tools := buildTools(claudeReq.Tools)
@@ -183,34 +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 {
-		log.Printf("[Debug] Signature too short: len=%d", len(signature))
-		return false
-	}
-
-	// 检查是否是有效的 base64 字符
-	// base64 字符集: A-Z, a-z, 0-9, +, /, =
-	for i, c := range signature {
-		if (c < 'A' || c > 'Z') && (c < 'a' || c > 'z') &&
-			(c < '0' || c > '9') && c != '+' && c != '/' && c != '=' {
-			log.Printf("[Debug] Invalid base64 character at position %d: %c (code=%d)", i, 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) {
@@ -239,30 +200,22 @@ func buildParts(content json.RawMessage, toolIDToName map[string]string, allowDu
 			}
 
 		case "thinking":
-			if allowDummyThought {
-				// Gemini 模型可以使用 dummy signature
-				parts = append(parts, GeminiPart{
-					Text:             block.Thinking,
-					Thought:          true,
-					ThoughtSignature: dummyThoughtSignature,
-				})
-				continue
+			part := GeminiPart{
+				Text:    block.Thinking,
+				Thought: true,
 			}
-
-			// 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)")
+			// 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
 			}
-			if !isValidThoughtSignature(signature) {
-				log.Printf("[Debug] Thinking signature may be invalid (passing through anyway): len=%d", len(signature))
-			}
-			parts = append(parts, GeminiPart{
-				Text:             block.Thinking,
-				Thought:          true,
-				ThoughtSignature: signature,
-			})
+			parts = append(parts, part)
 
 		case "image":
 			if block.Source != nil && block.Source.Type == "base64" {