Bump Petri to the commit that reports `pebble.usage_by_model`, the root
session's usage by the route that spent it, and add an end-to-end test:
an agent stage on a scripted model whose primary answers once and then
fails mid-prompt keeps every token Petri reported, and files the
primary's share and the fallback's share under their own models.
Only Petri moves; Pebble and sandbox-driver stay at their locked
revisions.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>