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_request_state.py

@@ -0,0 +1,32 @@
+from __future__ import annotations
+
+from src.services.task.request_state import MutableRequestBodyState
+
+
+def test_mutable_request_body_state_keeps_original_and_attempts_isolated() -> None:
+    original = {
+        "model": "gpt-5",
+        "input": [{"role": "user", "content": [{"type": "input_text", "text": "hello"}]}],
+    }
+
+    state = MutableRequestBodyState(original)
+
+    first_attempt = state.build_attempt_body()
+    first_attempt["input"][0]["content"][0]["text"] = "attempt-1"
+
+    assert original["input"][0]["content"][0]["text"] == "hello"
+    assert state.current_body["input"][0]["content"][0]["text"] == "hello"
+
+    rectified = state.build_attempt_body()
+    rectified["input"][0]["content"][0]["text"] = "rectified"
+    state.mark_rectified(rectified, stage=1)
+
+    second_attempt = state.build_attempt_body()
+    second_attempt["input"][0]["content"][0]["text"] = "attempt-2"
+
+    assert state.is_rectified() is True
+    assert state.rectify_stage() == 1
+    assert state.current_body["input"][0]["content"][0]["text"] == "rectified"
+    assert original["input"][0]["content"][0]["text"] == "hello"
+    assert state.consume_rectified_this_turn() is True
+    assert state.consume_rectified_this_turn() is False