feat: 流式处理统一返回用户/系统内容并完善消息存储

引入 StreamProcessResult 统一封装流式处理结果，补充各 API 类型下用户输入与系统输出内容的提取与累计，用于会话消息持久化与用量统计；同时增强 Gemini 请求与响应内容解析能力，确保流式场景下消息与 token 使用数据完整一致。

Yi.Abp.Net8/module/ai-hub/Yi.Framework.AiHub.Domain.Shared/Dtos/Gemini/GeminiGenerateContentAcquirer.cs

@@ -6,6 +6,83 @@ namespace Yi.Framework.AiHub.Domain.Shared.Dtos.Gemini;
 
 public static class GeminiGenerateContentAcquirer
 {
+    /// <summary>
+    /// 从请求体中提取用户最后一条消息内容
+    /// 路径: contents[last].parts[last].text
+    /// </summary>
+    public static string GetLastUserContent(JsonElement request)
+    {
+        var contents = request.GetPath("contents");
+        if (!contents.HasValue || contents.Value.ValueKind != JsonValueKind.Array)
+        {
+            return string.Empty;
+        }
+
+        var contentsArray = contents.Value.EnumerateArray().ToList();
+        if (contentsArray.Count == 0)
+        {
+            return string.Empty;
+        }
+
+        var lastContent = contentsArray[^1];
+        var parts = lastContent.GetPath("parts");
+        if (!parts.HasValue || parts.Value.ValueKind != JsonValueKind.Array)
+        {
+            return string.Empty;
+        }
+
+        var partsArray = parts.Value.EnumerateArray().ToList();
+        if (partsArray.Count == 0)
+        {
+            return string.Empty;
+        }
+
+        // 获取最后一个 part 的 text
+        var lastPart = partsArray[^1];
+        return lastPart.GetPath("text").GetString() ?? string.Empty;
+    }
+
+    /// <summary>
+    /// 从响应中提取文本内容（非 thought 类型）
+    /// 路径: candidates[0].content.parts[].text (where thought != true)
+    /// </summary>
+    public static string GetTextContent(JsonElement response)
+    {
+        var candidates = response.GetPath("candidates");
+        if (!candidates.HasValue || candidates.Value.ValueKind != JsonValueKind.Array)
+        {
+            return string.Empty;
+        }
+
+        var candidatesArray = candidates.Value.EnumerateArray().ToList();
+        if (candidatesArray.Count == 0)
+        {
+            return string.Empty;
+        }
+
+        var parts = candidatesArray[0].GetPath("content", "parts");
+        if (!parts.HasValue || parts.Value.ValueKind != JsonValueKind.Array)
+        {
+            return string.Empty;
+        }
+
+        // 遍历所有 parts，只取非 thought 的 text
+        foreach (var part in parts.Value.EnumerateArray())
+        {
+            var isThought = part.GetPath("thought").GetBool();
+            if (!isThought)
+            {
+                var text = part.GetPath("text").GetString();
+                if (!string.IsNullOrEmpty(text))
+                {
+                    return text;
+                }
+            }
+        }
+
+        return string.Empty;
+    }
+
     public static ThorUsageResponse? GetUsage(JsonElement response)
     {
         var usage = response.GetPath("usageMetadata");