refactor(backend): consolidate thread_id resolution into shared get_thread_id() utility (#2522)

Extract duplicated thread_id fallback logic from 11 files into a single
  deerflow.utils.runtime.get_thread_id() function with a documented 3-level
  cascade (runtime.context → runtime.config → get_config()).

  The module docstring also clarifies the __pregel_runtime injection pattern used in
  gateway mode.

backend/packages/harness/deerflow/utils/runtime.py

@@ -0,0 +1,88 @@
+"""Runtime utilities for thread_id resolution and context access.
+
+Thread ID Resolution Strategy
+=============================
+
+DeerFlow resolves the current ``thread_id`` from a three-level cascade:
+
+1. **runtime.context["thread_id"]** -- Set by ``worker.py`` (gateway mode)
+   or by LangGraph Server (standard mode) when constructing the Runtime.
+2. **runtime.config["configurable"]["thread_id"]** -- Available on
+   ``ToolRuntime`` instances passed to tools via the ``@tool`` decorator.
+   Not available on ``Runtime`` instances received by middlewares.
+3. **get_config()["configurable"]["thread_id"]** -- LangGraph's thread-local
+   config, available when executing inside a graph's runnable context.
+
+About ``__pregel_runtime``
+===========================
+
+In gateway mode (``run_agent()`` in ``worker.py``), the agent graph does not
+run inside the LangGraph Server.  The server normally injects a ``Runtime``
+object automatically.  Since we run the graph ourselves, we must inject the
+Runtime manually via ``config["configurable"]["__pregel_runtime"]``.  This is
+the standard mechanism provided by LangGraph's Pregel engine for injecting
+runtime context into graph nodes.  It is not a private/internal hack -- it is
+the documented way to pass Runtime when running a graph outside the server.
+
+Duck Typing
+===========
+
+Both ``langgraph.runtime.Runtime`` (middlewares) and
+``langchain.tools.ToolRuntime`` (tools) expose a ``.context`` attribute (a
+dict or None).  ``ToolRuntime`` additionally exposes ``.config``.  The
+function below uses ``getattr`` with safe defaults so it works with either
+type, with ``SimpleNamespace`` in tests, or with ``None``.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+
+def get_thread_id(runtime: Any | None) -> str | None:
+    """Resolve the current thread_id from a runtime object.
+
+    Follows a three-level fallback chain:
+
+    1. ``runtime.context.get("thread_id")`` -- if context is a non-empty dict.
+    2. ``runtime.config.get("configurable", {}).get("thread_id")`` -- if
+       the runtime has a config dict (ToolRuntime).
+    3. ``get_config().get("configurable", {}).get("thread_id")`` -- LangGraph's
+       thread-local config.  Wrapped in ``try/except RuntimeError`` because it
+       raises outside a runnable context (e.g., unit tests).
+
+    Args:
+        runtime: A Runtime, ToolRuntime, SimpleNamespace, or None.
+
+    Returns:
+        The thread_id string, or None if it cannot be resolved.
+    """
+    if runtime is None:
+        return None
+
+    # Level 1: runtime.context["thread_id"]
+    context = getattr(runtime, "context", None)
+    if context and isinstance(context, dict):
+        thread_id = context.get("thread_id")
+        if thread_id:
+            return thread_id
+
+    # Level 2: runtime.config["configurable"]["thread_id"]
+    config = getattr(runtime, "config", None)
+    if config and isinstance(config, dict):
+        thread_id = config.get("configurable", {}).get("thread_id")
+        if thread_id:
+            return thread_id
+
+    # Level 3: langgraph.config.get_config() -- only works inside runnable context
+    try:
+        from langgraph.config import get_config
+
+        config_data = get_config()
+        thread_id = config_data.get("configurable", {}).get("thread_id")
+        if thread_id:
+            return thread_id
+    except RuntimeError:
+        pass
+
+    return None