refactor(task): 引入 MutableRequestBodyState 替代 request_body_ref 字典容器

将请求体可变状态从 {"body": dict} 字典容器重构为独立的
MutableRequestBodyState 类，统一管理 original_body / current_body /
build_attempt_body / rectify 等语义，消除各层通过 ref["body"] 间接
读写的隐式约定。

- 新增 src/services/task/request_state.py 定义 Protocol 与实现
- handler/executor/mixin 层改用 request_state 参数传递
- error_handler/state_transition 通过 request_state 判断整流状态
- 新增 request_state 单元测试与 chat/cli 请求体隔离测试

tests/api/handlers/base/test_cli_request_body_isolation.py

@@ -10,6 +10,7 @@ import src.api.handlers.base.cli_request_mixin as request_mixmod
 from src.api.handlers.base.cli_request_mixin import CliRequestMixin
 from src.api.handlers.base.cli_stream_mixin import CliStreamMixin
 from src.api.handlers.base.stream_context import StreamContext
+from src.services.task.request_state import MutableRequestBodyState
 
 
 class _StopBuild(Exception):
@@ -135,6 +136,7 @@ async def test_execute_stream_request_does_not_mutate_original_request_body(
         ],
     }
     snapshot = copy.deepcopy(original_request_body)
+    request_state = MutableRequestBodyState(original_request_body)
 
     with pytest.raises(_StopBuild):
         await handler._execute_stream_request(
@@ -142,7 +144,7 @@ async def test_execute_stream_request_does_not_mutate_original_request_body(
             provider,
             endpoint,
             key,
-            original_request_body,
+            request_state.build_attempt_body(),
             {},
             candidate=candidate,
         )