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/engine/context/builder.py

@@ -80,6 +80,16 @@ class SessionContext:
     parent_session_id: str | None = None
 
 
+@dataclass(slots=True)
+class RuntimeContext:
+    """Per-run runtime facts that should be visible to the model."""
+
+    utc_datetime: str
+    local_datetime: str
+    timezone: str | None = None
+    utc_offset: str | None = None
+
+
 @dataclass(slots=True)
 class ContextBuildInput:
     """一次上下文构建所需的全部输入。
@@ -103,6 +113,7 @@ class ContextBuildInput:
     memory_snapshot: MemorySnapshot | None = None
     activated_skills: list[SkillContext] = field(default_factory=list)
     session_context: SessionContext | None = None
+    runtime_context: RuntimeContext | None = None
     execution_context: str | None = None
     extra_sections: list[str] = field(default_factory=list)
 
@@ -143,9 +154,10 @@ class ContextBuilder:
         1. Beaver user-facing assistant identity
         2. base system prompt
         3. session metadata
-        4. execution context
-        5. frozen memory snapshot
-        6. extra sections
+        4. runtime date/time
+        5. execution context
+        6. frozen memory snapshot
+        7. extra sections
 
         这样设计的原因：
         - 身份与总规则要最靠前
@@ -164,6 +176,10 @@ class ContextBuilder:
         if session_section:
             sections.append(session_section)
 
+        runtime_section = self._render_runtime_section(build_input.runtime_context)
+        if runtime_section:
+            sections.append(runtime_section)
+
         execution_context = (build_input.execution_context or "").strip()
         if execution_context:
             sections.append(f"# Execution Context\n\n{execution_context}")
@@ -347,6 +363,31 @@ class ContextBuilder:
             return None
         return "# Current Session\n\n" + "\n".join(rows)
 
+    def _render_runtime_section(self, runtime_context: RuntimeContext | None) -> str | None:
+        """Render date/time facts captured for the current model run."""
+
+        if runtime_context is None:
+            return None
+
+        rows: list[str] = []
+        if runtime_context.utc_datetime:
+            rows.append(f"Current UTC time: {runtime_context.utc_datetime}")
+        if runtime_context.local_datetime:
+            rows.append(f"Current local time: {runtime_context.local_datetime}")
+        if runtime_context.timezone:
+            rows.append(f"Local timezone: {runtime_context.timezone}")
+        if runtime_context.utc_offset:
+            rows.append(f"Local UTC offset: {runtime_context.utc_offset}")
+
+        if not rows:
+            return None
+        return (
+            "# Current Date and Time\n\n"
+            + "\n".join(rows)
+            + "\n\nUse this section as authoritative for relative date/time references such as "
+            '"today", "tomorrow", "now", "this week", and "next month".'
+        )
+
     def build_skill_activation_messages(self, activated_skills: list[SkillContext]) -> list[dict[str, str]]:
         """把已激活 skill 转成显式消息。