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.
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