`before_agent` guards `context = runtime.context or {}` at the top, but the
`run_id` stamp on a trailing HumanMessage still read the raw `runtime.context`,
so a None context (thread_id resolved from `config.configurable`) plus a
HumanMessage last message raised `AttributeError: 'NoneType' object has no
attribute 'get'`. Use the guarded local `context` instead.
Adds a regression test.