Pre-PR codex review flagged missing coverage for the 10 MB scanner buffer
called out in design §3.5. Adds:
- TestParseLastUsage_largeLine: 5 MB JSONL line with inert padding still
extracts the usage object correctly (no silent truncation).
- TestParseLastUsage_oversizedLineErrors: 11 MB line surfaces a scanner
error cleanly (no panic, nil usage).
Follow-up to parent 1.0, dev/plans/26174-planctl-context-tokens/tasks.md.