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_catalog.py

@@ -54,6 +54,23 @@ def test_search_invalid_regex_falls_back_to_literal():
     assert cat.search("zzz(") == []
 
 
+def test_search_empty_query_returns_empty(catalog):
+    # An empty / whitespace-only query is meaningless; rather than let the empty
+    # regex match every tool, search() returns nothing so the model gets a clear
+    # "no match" signal and re-queries instead of acting on noise.
+    assert catalog.search("") == []
+    assert catalog.search("   ") == []
+
+
+def test_search_bare_plus_returns_empty(catalog):
+    # A "+" prefix with no required token is malformed model input. It must
+    # return no matches, not raise IndexError on parts[0]. " + " strips to "+",
+    # so it routes here too and must be handled the same way.
+    assert catalog.search("+") == []
+    assert catalog.search(" + ") == []
+    assert catalog.search("+   ") == []
+
+
 def test_hash_stable_across_instances():
     c1 = DeferredToolCatalog((alpha_search, beta_translate))
     c2 = DeferredToolCatalog((beta_translate, alpha_search))