refactor: thread app_config through middleware factories (#2652)

* refactor: thread app_config through middleware factories

Continues the incremental config-refactor sequence (#2611 root, #2612 lead
path) one layer deeper into the middleware factories. Two ambient lookups
inside _build_runtime_middlewares are eliminated and the LLMErrorHandling
band-aid removed:

- _build_runtime_middlewares / build_lead_runtime_middlewares /
  build_subagent_runtime_middlewares now require app_config: AppConfig.
- get_guardrails_config() inside the factory is replaced with
  app_config.guardrails (semantically identical — same default-factory
  GuardrailsConfig — verified by direct equality check).
- LLMErrorHandlingMiddleware.__init__ now requires app_config and reads
  circuit_breaker fields directly. The class-level
  circuit_failure_threshold / circuit_recovery_timeout_sec defaults are
  removed along with the try/except (FileNotFoundError, RuntimeError):
  pass band-aid — the let-it-crash invariant the rest of the refactor
  enforces.

Caller chain (already-resolved app_config sources):
- _build_middlewares in lead_agent/agent.py: reorder so
  resolved_app_config = app_config or get_app_config() is computed BEFORE
  build_lead_runtime_middlewares is called, then passed as kwarg.
- SubagentExecutor: optional app_config parameter (mirrors the lead-agent
  pattern); _create_agent does the same `or get_app_config()` fallback at
  agent-build time, so task_tool callers don't need to plumb app_config
  through yet (typed-context plumbing for tool runtimes is a separate
  refactor).

Tests:
- test_llm_error_handling_middleware: _make_app_config helper using
  AppConfig(sandbox=SandboxConfig(use="test")) — same minimal-config
  pattern conftest already uses. Three direct LLMErrorHandlingMiddleware()
  calls each followed by post-construction circuit_breaker mutation fold
  cleanly into _build_middleware(circuit_failure_threshold=...,
  circuit_recovery_timeout_sec=...).

Verification:
- tests/test_llm_error_handling_middleware.py — 14 passed
- tests/test_subagent_executor.py — 28 passed
- tests/test_tool_error_handling_middleware.py — 6 passed
- tests/test_task_tool_core_logic.py — 18 passed (verifies task_tool
  unchanged behavior)
- Full suite: 2697 passed, 3 skipped. The single intermittent failure in
  tests/test_client_e2e.py::test_tool_call_produces_events is pre-existing
  LLM flakiness (the test asserts the model decided to call a tool;
  reproduces 1/3 on unchanged main as well).

* fix: address middleware app config review comments

* fix: satisfy app config annotation lint

* test: cover explicit app config middleware wiring

---------

Co-authored-by: greatmengqi <chenmengqi.0376@bytedance.com>

backend/tests/test_llm_error_handling_middleware.py

@@ -11,6 +11,13 @@ from langgraph.errors import GraphBubbleUp
 from deerflow.agents.middlewares.llm_error_handling_middleware import (
     LLMErrorHandlingMiddleware,
 )
+from deerflow.config.app_config import AppConfig
+from deerflow.config.sandbox_config import SandboxConfig
+
+
+def _make_app_config() -> AppConfig:
+    """Minimal AppConfig for middleware tests; circuit_breaker uses defaults."""
+    return AppConfig(sandbox=SandboxConfig(use="test"))
 
 
 class FakeError(Exception):
@@ -31,7 +38,7 @@ class FakeError(Exception):
 
 
 def _build_middleware(**attrs: int) -> LLMErrorHandlingMiddleware:
-    middleware = LLMErrorHandlingMiddleware()
+    middleware = LLMErrorHandlingMiddleware(app_config=_make_app_config())
     for key, value in attrs.items():
         setattr(middleware, key, value)
     return middleware
@@ -226,9 +233,7 @@ def test_circuit_breaker_trips_and_recovers(monkeypatch: pytest.MonkeyPatch) ->
     current_time = 1000.0
     monkeypatch.setattr("time.time", lambda: current_time)
 
-    middleware = LLMErrorHandlingMiddleware()
-    middleware.circuit_failure_threshold = 3
-    middleware.circuit_recovery_timeout_sec = 10
+    middleware = _build_middleware(circuit_failure_threshold=3, circuit_recovery_timeout_sec=10)
     monkeypatch.setattr(middleware, "_classify_error", mock_classify_retriable)
 
     request: Any = {"messages": []}
@@ -284,8 +289,7 @@ def test_circuit_breaker_does_not_trip_on_non_retriable_errors(monkeypatch: pyte
     waits: list[float] = []
     monkeypatch.setattr("time.sleep", lambda d: waits.append(d))
 
-    middleware = LLMErrorHandlingMiddleware()
-    middleware.circuit_failure_threshold = 3
+    middleware = _build_middleware(circuit_failure_threshold=3)
     monkeypatch.setattr(middleware, "_classify_error", mock_classify_non_retriable)
 
     request: Any = {"messages": []}
@@ -386,9 +390,7 @@ async def test_async_circuit_breaker_trips_and_recovers(monkeypatch: pytest.Monk
     current_time = 1000.0
     monkeypatch.setattr("time.time", lambda: current_time)
 
-    middleware = LLMErrorHandlingMiddleware()
-    middleware.circuit_failure_threshold = 3
-    middleware.circuit_recovery_timeout_sec = 10
+    middleware = _build_middleware(circuit_failure_threshold=3, circuit_recovery_timeout_sec=10)
     monkeypatch.setattr(middleware, "_classify_error", mock_classify_retriable)
 
     async def async_failing_handler(request: Any) -> Any: