fix: Claude 流式断流时不再整份覆盖 usage，保留 cache 计费字段

HandleStreamFinalResponse 在 !Done 时调用 ResponseText2Usage 整份覆盖
claudeInfo.Usage，导致 message_start 已获取的 CacheReadInputTokens、
CacheCreationInputTokens 等字段丢失，prompt 退化为占位值 1。

修复：
- 只补缺失的 CompletionTokens/PromptTokens，保留已有 cache 数据
- PromptTokens 兜底改用 info.GetEstimatePromptTokens()（与其他渠道对齐）

Fixes #4127

relay/channel/claude/relay-claude.go

@@ -809,7 +809,15 @@ func HandleStreamFinalResponse(c *gin.Context, info *relaycommon.RelayInfo, clau
 		if common.DebugEnabled {
 			common.SysLog("claude response usage is not complete, maybe upstream error")
 		}
-		claudeInfo.Usage = service.ResponseText2Usage(c, claudeInfo.ResponseText.String(), info.UpstreamModelName, claudeInfo.Usage.PromptTokens)
+		// 只补缺失字段，不整份覆盖——保留 message_start 已拿到的 cache 字段
+		fallback := service.ResponseText2Usage(c, claudeInfo.ResponseText.String(), info.UpstreamModelName, info.GetEstimatePromptTokens())
+		if claudeInfo.Usage.CompletionTokens == 0 {
+			claudeInfo.Usage.CompletionTokens = fallback.CompletionTokens
+		}
+		if claudeInfo.Usage.PromptTokens == 0 {
+			claudeInfo.Usage.PromptTokens = fallback.PromptTokens
+		}
+		claudeInfo.Usage.TotalTokens = claudeInfo.Usage.PromptTokens + claudeInfo.Usage.CompletionTokens
 	}
 	if claudeInfo.Usage != nil {
 		claudeInfo.Usage.UsageSemantic = "anthropic"