The task tool picked the last text part of the subagent's final message as
its result. Subagents that ran with memory context get a synthetic, ignored,
empty text part (the memory marker) appended after their answer, so findLast
surfaced an empty <task_result> to the parent agent. The extractor now skips
synthetic, ignored, and empty text parts, and background jobs no longer let
an empty run overwrite an earlier non-empty result on extend.
Fixes#13469
maphew
bf7848cb48