* fix(gateway): reject unusable tool_artifacts entries at the input boundary
merge_tool_artifacts keys every registry entry by handle, so a caller-supplied
entry with a missing or unhashable handle raised while the state update
merged; the thread-state route persists reducer values with Overwrite, so the
bad shape was stored and every later artifact capture failed on it. Validate
the list-of-objects shape with a non-empty string handle on both boundaries,
and skip such an entry in the reducer so a poisoned checkpoint heals.
* fix(gateway): keep the per-entry metadata strip for tool_artifacts
The dedicated registry branch replaced the generic list branch and silently
dropped its _strip_external_metadata_from_message_like pass. Registry entries
are rich dicts, so a caller-crafted entry could persist server-owned message
metadata into the checkpointed registry. Apply the strip to each validated
entry; it is a no-op for legitimate registry entries.
599105e0fb