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/packages/harness/deerflow/tools/tools.py

@@ -7,6 +7,7 @@ from deerflow.config.app_config import AppConfig
 from deerflow.reflection import resolve_variable
 from deerflow.sandbox.security import is_host_bash_allowed
 from deerflow.tools.builtins import ask_clarification_tool, present_file_tool, task_tool, view_image_tool
+from deerflow.tools.mcp_metadata import tag_mcp_tool
 from deerflow.tools.sync import make_sync_tool_wrapper
 
 logger = logging.getLogger(__name__)
@@ -132,7 +133,7 @@ def get_available_tools(
                     # the deferred catalog + tool_search tool are assembled per
                     # agent from the policy-filtered tool list.
                     for t in mcp_tools:
-                        t.metadata = {**(t.metadata or {}), "deerflow_mcp": True}
+                        tag_mcp_tool(t)
         except ImportError:
             logger.warning("MCP module not available. Install 'langchain-mcp-adapters' package to enable MCP tools.")
         except Exception as e: