fix: Message.ReasoningContent/Reasoning 改为 *string，修复空思考内容在请求转发时被静默丢弃的问题

问题：
在非 passThrough 模式下，客户端发送的 reasoning_content: "" 经过
Go struct 反序列化再序列化后，因 string + omitempty 无法区分空串和
字段缺失，导致空的思考内容被静默丢弃。

根因：
dto.Message.ReasoningContent 和 Message.Reasoning 使用 string（非指针）
加 omitempty，违反 AGENTS.md Rule 6（可选标量字段必须用指针类型）。

修复：
1. Message.ReasoningContent/Reasoning 类型从 string 改为 *string
   - nil = 字段缺失 → JSON 省略
   - &"" = 显式空串 → JSON 保留 reasoning_content: ""
2. 新增 Message.GetReasoningContent() 辅助方法
3. 更新所有读写处：relay-openai, relay-claude, relay-gemini, ollama
4. 新增测试覆盖空串保留、字段省略、getter 回退逻辑

relay/channel/claude/relay-claude.go

@@ -567,12 +567,14 @@ func ResponseClaude2OpenAI(claudeResponse *dto.ClaudeResponse) *dto.OpenAITextRe
 	}
 	choice.SetStringContent(responseText)
 	if len(responseThinking) > 0 {
-		choice.ReasoningContent = responseThinking
+		choice.ReasoningContent = &responseThinking
 	}
 	if len(tools) > 0 {
 		choice.Message.SetToolCalls(tools)
 	}
-	choice.Message.ReasoningContent = thinkingContent
+	if thinkingContent != "" {
+		choice.Message.ReasoningContent = &thinkingContent
+	}
 	fullTextResponse.Model = claudeResponse.Model
 	choices = append(choices, choice)
 	fullTextResponse.Choices = choices

relay/channel/gemini/relay-gemini.go

@@ -1097,7 +1097,7 @@ func responseGeminiChat2OpenAI(c *gin.Context, response *dto.GeminiChatResponse)
 						toolCalls = append(toolCalls, *call)
 					}
 				} else if part.Thought {
-					choice.Message.ReasoningContent = part.Text
+					choice.Message.ReasoningContent = &part.Text
 				} else {
 					if part.ExecutableCode != nil {
 						texts = append(texts, "```"+part.ExecutableCode.Language+"\n"+part.ExecutableCode.Code+"\n```")

relay/channel/ollama/stream.go

@@ -273,7 +273,7 @@ func ollamaChatHandler(c *gin.Context, info *relaycommon.RelayInfo, resp *http.R
 
 	msg := dto.Message{Role: "assistant", Content: contentPtr(content)}
 	if rc := reasoningBuilder.String(); rc != "" {
-		msg.ReasoningContent = rc
+		msg.ReasoningContent = &rc
 	}
 	full := dto.OpenAITextResponse{
 		Id:      common.GetUUID(),

relay/channel/openai/relay-openai.go

@@ -245,7 +245,7 @@ func OpenaiHandler(c *gin.Context, info *relaycommon.RelayInfo, resp *http.Respo
 		completionTokens := simpleResponse.Usage.CompletionTokens
 		if completionTokens == 0 {
 			for _, choice := range simpleResponse.Choices {
-				ctkm := service.CountTextToken(choice.Message.StringContent()+choice.Message.ReasoningContent+choice.Message.Reasoning, info.UpstreamModelName)
+				ctkm := service.CountTextToken(choice.Message.StringContent()+choice.Message.GetReasoningContent(), info.UpstreamModelName)
 				completionTokens += ctkm
 			}
 		}