feat(services): assignment 6 skeleton - app service, field registry, in-memory repository

- fields_registry: 24 business keys in acd/human1/human2 groups with
  value_type, writable and SensitiveLevel metadata; strict bool/int
  parsing (no bool('0') trap); INTERNAL_FIELD_KEYS protection list
- repository: SessionRepository protocol + InMemorySessionRepository
  (idempotent delete; PostgreSQL swap-in is assignment 7)
- AccidentAppService: chat/set_info/get_info/delete_session share one
  business state - chat injects latest repository fields into the graph
  and writes back validated field diffs; set/get/delete make zero LLM
  calls and zero helper conversations (includeInputInfo accepted but
  unused); unknown get keys keep the '' JSON-empty-string contract,
  unknown set keys are rejected by whitelist
- ChatInput.injected_fields neutral contract extension; LangGraphBackend
  merges injected fields into graph input (set_info visible next chat)
- endpoints: thin layer - langgraph mode routes business state through
  the app service (fastgpt mode keeps the legacy path); delete cleans
  repository + graph checkpoint, idempotent
- tests: registry completeness (sfzmwh/sjwh/csbw included), app service
  combos with LLM-call spy, HTTP acceptance flow (set -> get -> chat ->
  groups -> delete -> no leak, idempotent re-delete)
- docs/assignment6-compat.md: behavior diff list vs FastGPT status quo
  and the audit backlog (arbitrary set_info key usage, stream formUpdate
  write-back, sensitive-field display boundary)

Verified: 200 unit tests (172 existing + 28 new)

src/services/repository.py

@@ -0,0 +1,52 @@
+"""会话业务状态 repository（作业 6-1/6-2）。
+
+抽象出 repository 接口，使应用服务不依赖具体存储；
+作业 6 用内存实现跑通四接口，作业 7 再替换为 PostgreSQL 实现
+（对话 checkpoint 已由 LangGraph checkpointer 承担，本层只管
+业务字段——单一事实来源，图每轮读取最新值）。
+
+语义约定：
+- 业务状态按外部业务键（hphm1、ywrysw...）扁平存储，值为内部强类型；
+- 不存在的会话视为空状态（读取返回 {}），与首轮 checkpoint 无字段一致；
+- delete 幂等：重复删除固定成功，不抛异常（行为进测试）。
+"""
+
+from typing import Protocol
+
+
+class SessionRepository(Protocol):
+    """业务字段存储接口（作业 7 由 PostgreSQL 实现替换）。"""
+
+    def load(self, session_id: str) -> dict:
+        """读取会话业务字段；无记录时返回空 dict。"""
+        ...
+
+    def apply_update(self, session_id: str, patch: dict) -> dict:
+        """合并差量并返回合并后的完整业务字段。"""
+        ...
+
+    def delete(self, session_id: str) -> None:
+        """删除会话业务状态；幂等，重复删除不抛异常。"""
+        ...
+
+
+class InMemorySessionRepository:
+    """内存实现：字典存储，仅用于单进程开发与测试。"""
+
+    def __init__(self) -> None:
+        self._store: dict[str, dict] = {}
+
+    def load(self, session_id: str) -> dict:
+        return dict(self._store.get(session_id) or {})
+
+    def apply_update(self, session_id: str, patch: dict) -> dict:
+        state = self._store.setdefault(session_id, {})
+        state.update(patch)
+        return dict(state)
+
+    def delete(self, session_id: str) -> None:
+        # 幂等：pop 的缺省值吞掉不存在的情况，重复删除行为固定为成功
+        self._store.pop(session_id, None)
+
+
+__all__ = ["SessionRepository", "InMemorySessionRepository"]