refactor(tool-search): consolidate MCP metadata tag and harden deferred-tool setup (#3370)

Follow-up to #3342 (deferred MCP tool loading). Maintainability cleanup plus
hardening of malformed/empty tool_search queries; no change to the deferral
mechanism or search ranking.

- Add deerflow/tools/mcp_metadata.py as the single source of truth for the
  "deerflow_mcp" tag (MCP_TOOL_METADATA_KEY + tag_mcp_tool + public
  is_mcp_tool). Removes the duplicated magic string and the private,
  cross-module _is_mcp_tool import.
- tool_search.search: never raise on model-generated input. Extract
  _compile_catalog_regex (shared compile-with-literal-fallback); return empty
  for empty/whitespace queries and a bare "+" instead of matching everything
  or raising IndexError.
- DeferredToolSetup: document the empty-vs-populated invariant.
- build_deferred_tool_setup: comment the two distinct empty-return branches.
- _assemble_deferred: add return type, rename local to deferred_setup, build
  the final list with an explicit append.
- Tests: use tag_mcp_tool instead of per-file tag helpers; cover empty and
  bare-"+" queries.

backend/tests/test_deferred_promotion_integration.py

@@ -20,6 +20,7 @@ from langchain_core.tools import tool as as_tool
 from deerflow.agents.middlewares.deferred_tool_filter_middleware import DeferredToolFilterMiddleware
 from deerflow.agents.thread_state import ThreadState
 from deerflow.tools.builtins.tool_search import build_deferred_tool_setup
+from deerflow.tools.mcp_metadata import tag_mcp_tool
 
 
 @as_tool
@@ -40,11 +41,6 @@ def mcp_other(x: str) -> str:
     return x
 
 
-def _tag(t):
-    t.metadata = {**(t.metadata or {}), "deerflow_mcp": True}
-    return t
-
-
 def test_tool_search_promotes_into_next_turn():
     bound: list[list[str]] = []
 
@@ -53,7 +49,7 @@ def test_tool_search_promotes_into_next_turn():
             bound.append([getattr(t, "name", None) for t in tools])
             return self
 
-    setup = build_deferred_tool_setup([active_tool, _tag(mcp_calc), _tag(mcp_other)], enabled=True)
+    setup = build_deferred_tool_setup([active_tool, tag_mcp_tool(mcp_calc), tag_mcp_tool(mcp_other)], enabled=True)
     turn1 = AIMessage(content="", tool_calls=[{"name": "tool_search", "args": {"query": "select:mcp_calc"}, "id": "c1", "type": "tool_call"}])
     turn2 = AIMessage(content="done")
     model = RecordingModel(messages=iter([turn1, turn2]))