refactor(lead-agent): make build_middlewares public to drop the last cross-module private import (#3458)

`client.py` imported the private `_build_middlewares` from `agent.py` across a
module boundary and called it as public API. Because the `_` name signals
"module-private, no external callers", any future rename or signature change
silently breaks the embedded `DeerFlowClient` path — and the test suite even
monkeypatched `deerflow.client._build_middlewares`, baking the leak in.

`DeerFlowClient` is a lead-agent variant that genuinely needs the lead agent's
full middleware composition, so make the dependency honest: promote the helper
to a documented public entry point `build_middlewares` and update every in-repo
caller. Found during #3341 review; #3341 already removed one such leak
(`_assemble_deferred` -> public `assemble_deferred_tools`) and left this one out
of scope on purpose.

- agent.py: rename def + both internal call sites; expand the docstring into a
  public-entry-point contract and document the previously-undocumented
  model_name / app_config / deferred_setup params
- client.py: import + call site now use the public name (removes the last
  cross-module private import)
- scripts/tool-error-degradation-detection.sh: update its import + call site
- tests (5 files): update monkeypatch/patch targets and direct calls
- docs (backend/CLAUDE.md, plan_mode_usage.md, middlewares.mdx): sync the live
  references that describe the symbol as current API

Pure mechanical rename, no behavior change. Historical design docs (rfc,
superpowers spec) intentionally keep the old name as point-in-time records.

Closes #3431

scripts/tool-error-degradation-detection.sh

@@ -35,7 +35,7 @@ from requests.exceptions import SSLError
 from langchain.agents.middleware import AgentMiddleware
 from langchain_core.messages import ToolMessage
 
-from deerflow.agents.lead_agent.agent import _build_middlewares
+from deerflow.agents.lead_agent.agent import build_middlewares
 from deerflow.config import get_app_config
 from deerflow.sandbox.middleware import SandboxMiddleware
 
@@ -188,7 +188,7 @@ if not model_name:
     print("[FAIL] No model configured; cannot evaluate lead middleware chain.")
     raise SystemExit(8)
 
-lead_middlewares = _build_middlewares({"configurable": {}}, model_name=model_name)
+lead_middlewares = build_middlewares({"configurable": {}}, model_name=model_name)
 sub_middlewares = _build_sub_middlewares()
 
 print("[STEP 3] Simulate two sequential tool calls and check whether conversation flow aborts.")