* fix(ui): prioritize displayName over label in webchat session picker
The session picker dropdown in the webchat UI was showing raw session
keys instead of human-readable display names. resolveSessionDisplayName()
checked label before displayName and formatted displayName-based entries
as key (displayName) instead of displayName (key).
Swap the priority so displayName is checked first, and use a consistent
humanName (key) format for both displayName and label fallbacks.
Fixes#6645
* test: use deterministic updatedAt in session display name tests
137b7d9aab