refactor(agent): inject deps via context_schema, LLM-driven photo/confirm nodes

- graph.py: LangGraph-native DI - StateGraph(context_schema=GraphDeps),
  nodes take (state, runtime: Runtime[GraphDeps]); build_graph() decoupled
  from LLM; deleted scaffold _event_reply/_fake_llm_propose/stub nodes
- photo.py: model proposes stage+body via per-mode prompts (direct/addon);
  validator enforces clientMode exits; deterministic event forcing removed
- confirm.py: single runtime for single/dual confirm, extraction call
  isolated per person_index
- llm.py: workflow_fake_llm() (offline workflow-faithful responder,
  history-aware) for smoke/tests
- backend: LangGraphBackend(graph, llm) passes context per call
- tests: invoke helpers inject GraphDeps; illegal-proposal test asserts
  error + state untouched via second well-behaved fake

Verified: 143 passed; smoke turns 1/2/1 with original welcome text

test/agent/test_collect_llm.py

@@ -26,7 +26,8 @@ from src.agent.prompts import COLLECTION, load_prompt
 def collect_state(**overrides) -> dict:
     state = {
         "session_id": "s1",
-        "input_text": "你好",
+        # 使用完整句而非短促词，避免触发分流守卫（“你好”<=2 字）
+        "input_text": "发生了交通事故，两辆车追尾",
         "stage_code": "1002",
         "turn_count": 1,
         "no_reply_count": 0,
@@ -138,13 +139,16 @@ async def test_collect_illegal_stage_proposal_not_committed():
 
 @pytest.mark.asyncio
 async def test_graph_collect_node_with_llm():
-    # 图级：build_graph(llm=...) 时首轮走 welcome（不调模型），
-    # 第二轮起 collect 节点走 run_collect
+    # 图级：首轮走 welcome（确定性，不调模型）；第二轮起 collect
+    # 节点经 Runtime 注入的 llm 走 run_collect
+    from src.agent.graph import GraphDeps
+
     llm = FakeLLMService(lambda s, u: "<state>1002</state>收到。")
-    graph = build_graph(llm=llm)
+    graph = build_graph()
     first = await graph.ainvoke(
         {"session_id": "s1", "input_text": "追尾了"},
         config={"configurable": {"thread_id": "s1"}},
+        context=GraphDeps(llm=llm),
     )
     assert len(llm.calls) == 0  # welcome 是确定性节点
     assert first["stage_code"] == "1001"
@@ -152,6 +156,7 @@ async def test_graph_collect_node_with_llm():
     result = await graph.ainvoke(
         {"session_id": "s1", "input_text": "继续办理"},
         config={"configurable": {"thread_id": "s1"}},
+        context=GraphDeps(llm=llm),
     )
     assert len(llm.calls) == 1
     assert result["response_text"].startswith("<state>1002</state>")