feat(engine): 添加运行时上下文支持并重构工具迭代限制

添加 RuntimeContext 类用于捕获模型运行时的日期时间信息，
包括UTC时间、本地时间和时区信息，并在系统提示中显示这些信息。

同时增加最大上下文消息数和工具迭代次数的配置选项，
将验证服务从引擎加载器中移除，并更新相关的数据结构和接口。

BREAKING CHANGE: 移除了验证服务，相关字段被替换为证据状态和接受状态。

- 添加 RuntimeContext 类和相关渲染方法
- 增加 max_context_messages 和 max_tool_iterations 配置
- 移除 ValidationService 相关代码
- 更新消息记录中的验证状态字段
- 添加原始工具调用检测和回退处理

app-instance/backend/beaver/tasks/planner.py

@@ -10,7 +10,7 @@ from typing import Any, Literal
 from beaver.coordinator.models import AgentDescriptor, ExecutionGraph, ExecutionNode
 from beaver.engine.providers import ProviderBundle
 
-from .models import TaskRecord, ValidationResult
+from .models import TaskRecord
 from .skill_resolver import SkillResolutionReport, TaskSkillResolver
 
 
@@ -76,7 +76,6 @@ class TaskExecutionPlanner:
         task: TaskRecord,
         user_message: str,
         attempt_index: int,
-        latest_validation: ValidationResult | None = None,
         provider_bundle: ProviderBundle | None = None,
         timeout_seconds: float = 30.0,
     ) -> TaskExecutionPlan:
@@ -105,7 +104,6 @@ class TaskExecutionPlanner:
                                 task=task,
                                 user_message=user_message,
                                 attempt_index=attempt_index,
-                                latest_validation=latest_validation,
                             ),
                         },
                     ],
@@ -230,14 +228,10 @@ class TaskExecutionPlanner:
         task: TaskRecord,
         user_message: str,
         attempt_index: int,
-        latest_validation: ValidationResult | None,
     ) -> str:
-        validation_note = ""
-        if latest_validation is not None:
-            validation_note = (
-                "\nPrevious validation issues:\n"
-                + json.dumps(latest_validation.to_dict(), ensure_ascii=False)
-            )
+        history_note = ""
+        if task.feedback:
+            history_note = "\nRelevant task history:\n" + json.dumps(task.feedback[-5:], ensure_ascii=False)
         return (
             "Decide execution mode for this internal Task attempt.\n"
             "Use mode=team only when independent research, review, implementation slices, or staged checks "
@@ -254,7 +248,7 @@ class TaskExecutionPlanner:
             f"Task goal:\n{task.goal}\n\n"
             f"Current user request:\n{user_message}\n\n"
             f"Attempt index: {attempt_index}\n"
-            f"{validation_note}"
+            f"{history_note}"
         )
 
     @staticmethod