feat(agent): form pipeline + collection prompt workflow@20261021

- merge main: workflow/20261021 (collection prompt adds step-5 mandatory
  time readback-confirm before routing; non-motor/0003 branch rework;
  accident_cause unified schema), updated eval suites (photo_skip/
  button_guidance rewritten, no_false trimmed to asr, new
  non_motor_vehicle + time_confirmation with form/reply assertions),
  runner gains form/formMatches/replyMatches expectations
- prompts/collection.md regenerated from 20261021; extraction prompt
  archived (contentExtract description, 6-key whitelist)
- collect: extraction call per turn (extraction.md + current time/form
  injected), merge follows 更新表单 semantics (meaningful values only,
  number/string equality, diff -> turn_patch); accident_fields now uses
  external form keys
- backend: form_update flows to ChatResult and SSE formUpdate event
  (stream emits prefix, FormUpdate, body)
- fields: FORM_FIELDS whitelist with strict type validation
- verified: 155 unit tests; golden evals on real model - photo_skip 4/4,
  no_false 2/2, photo_button_guidance 12/12, non_motor_vehicle 14/14,
  collection_skip 10/12, time_confirmation 1/2; remaining 3 failures are
  model sequence/literal-template adherence, recorded in plan appendix

src/backends/langgraph.py

@@ -15,7 +15,14 @@
 from collections.abc import AsyncIterator
 
 from ..agent.graph import GraphDeps, build_graph, workflow_fake_llm
-from .chat import ChatInput, ChatResult, ChatStreamEvent, ChatBackend, TextDelta
+from .chat import (
+    ChatBackend,
+    ChatInput,
+    ChatResult,
+    ChatStreamEvent,
+    FormUpdate,
+    TextDelta,
+)
 
 
 class LangGraphBackend(ChatBackend):
@@ -54,7 +61,7 @@ class LangGraphBackend(ChatBackend):
         return ChatResult(
           content = result["response_text"],
           status_code = None,
-          form_update = {}
+          form_update = result.get("turn_patch") or {}
         )
 
     async def stream(self, chat_input: ChatInput) -> AsyncIterator[ChatStreamEvent]:
@@ -67,7 +74,12 @@ class LangGraphBackend(ChatBackend):
           “尚未接入模型增量输出”（作业 8 改为真实增量）。
         """
         result = await self.complete(chat_input)
-        prefix, _, body = result.content.partition("</state>")
+        prefix, sep, body = result.content.partition("</state>")
+        if not sep:
+            yield TextDelta(result.content)
+            return
         yield TextDelta(prefix + "</state>")
+        if result.form_update:
+            yield FormUpdate(result.form_update)
         if body:
             yield TextDelta(body)