perf(core): optimize callback manager hot paths
- get_child(): pass state directly to constructor instead of calling
set_handlers/add_tags/add_metadata sequentially (-22%)
- add_tags(): use set-based dedup instead of O(n) list scans per tag,
early return when tags list is empty (-70% on duplicate tags)
- handle_event/ahandle_event: early return when handlers list is empty
- _configure(): skip throwaway CallbackManager construction on the
common path where inheritable_callbacks is provided
These are the top bottlenecks identified by profiling langgraph's
react_agent benchmark, where langchain_core callbacks account for ~35%
of runtime.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
