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)

test/services/test_fields_registry.py

@@ -0,0 +1,89 @@
+"""作业 6：字段注册表测试（分组完整性、类型/敏感级别、外部表示）。"""
+
+import pytest
+
+from src.services.fields_registry import (
+    FIELD_REGISTRY,
+    GROUP_KEYS,
+    INTERNAL_FIELD_KEYS,
+    FieldGroup,
+    SensitiveLevel,
+    parse_field_value,
+    to_external_string,
+)
+
+
+def test_registry_covers_all_three_groups():
+    """注册表完整覆盖 acd/human1/human2 三个分组（作业 6-3）。"""
+    assert set(GROUP_KEYS) == set(FieldGroup)
+    assert len(FIELD_REGISTRY) == 24
+    assert len(GROUP_KEYS[FieldGroup.ACD]) == 8
+    assert len(GROUP_KEYS[FieldGroup.HUMAN1]) == 8
+    assert len(GROUP_KEYS[FieldGroup.HUMAN2]) == 8
+
+
+def test_person_fields_include_full_set():
+    """人员字段必须包含 sfzmwh/sjwh/csbw，不能只迁姓名和号码。"""
+    for index in (1, 2):
+        group_keys = set(GROUP_KEYS[FieldGroup.HUMAN1 if index == 1 else FieldGroup.HUMAN2])
+        for suffix in ("xm", "hpzl", "hphm", "sfzmhm", "sfzmwh", "sjhm", "sjwh", "csbw"):
+            assert f"{suffix}{index}" in group_keys
+
+
+def test_registry_metadata_recorded():
+    """每个字段都记录类型、可写性、敏感级别（作业 6-4）。"""
+    for spec in FIELD_REGISTRY.values():
+        assert spec.value_type in (str, bool, int)
+        assert spec.writable is True
+        assert isinstance(spec.sensitive, SensitiveLevel)
+        assert spec.description
+
+
+def test_sensitive_levels_assigned():
+    """证件号/手机号 sensitive，姓名 personal，其余 public。"""
+    assert FIELD_REGISTRY["sfzmhm1"].sensitive is SensitiveLevel.SENSITIVE
+    assert FIELD_REGISTRY["sjhm2"].sensitive is SensitiveLevel.SENSITIVE
+    assert FIELD_REGISTRY["xm1"].sensitive is SensitiveLevel.PERSONAL
+    assert FIELD_REGISTRY["hphm1"].sensitive is SensitiveLevel.PUBLIC
+
+
+@pytest.mark.parametrize("raw,expected", [
+    ("1", True), ("true", True), ("是", True),
+    ("0", False), ("false", False), ("否", False),
+])
+def test_parse_bool_explicit(raw, expected):
+    assert parse_field_value(FIELD_REGISTRY["ywrysw"], raw) is expected
+
+
+@pytest.mark.parametrize("bad", ["", "maybe", "2", "有吧"])
+def test_parse_bool_rejects_ambiguous(bad):
+    """模糊值必须拒绝——保留 空/False/0/未知 的区分能力。"""
+    with pytest.raises(ValueError):
+        parse_field_value(FIELD_REGISTRY["ywrysw"], bad)
+
+
+def test_parse_int_and_str():
+    assert parse_field_value(FIELD_REGISTRY["jdcsl"], "2") == 2
+    assert parse_field_value(FIELD_REGISTRY["jdcsl"], "0") == 0
+    with pytest.raises(ValueError):
+        parse_field_value(FIELD_REGISTRY["jdcsl"], "两辆")
+    assert parse_field_value(FIELD_REGISTRY["hphm1"], "苏B12345") == "苏B12345"
+
+
+def test_external_string_representation():
+    """对外序列化：True->1，False->0，None->空串；内部仍区分 False 与 0。"""
+    assert to_external_string(True) == "1"
+    assert to_external_string(False) == "0"
+    assert to_external_string(0) == "0"
+    assert to_external_string(None) == ""
+    assert to_external_string("苏B12345") == "苏B12345"
+    # 内部存储不混淆：False 是 bool、0 是 int
+    assert parse_field_value(FIELD_REGISTRY["ywrysw"], "0") is False
+    assert parse_field_value(FIELD_REGISTRY["jdcsl"], "0") == 0
+    assert isinstance(parse_field_value(FIELD_REGISTRY["jdcsl"], "0"), int)
+
+
+def test_internal_field_protection_listed():
+    """图内部键登记在保护清单中，set_info 白名单不会收录它们。"""
+    for key in INTERNAL_FIELD_KEYS:
+        assert key not in FIELD_REGISTRY