Expand backend switching test coverage

[DorianZheng]

Summary

• Add focused E2E coverage for user-turn backend handoff consumption, partial handoff fallback, failed first post-switch runs, and stale old-backend completions after a mid-run switch.
• Add scheduled-task E2E coverage for group-context handoff consumption and stale scheduled completions after backend switches.
• Extend UT coverage for registry migration, model override clearing, fresh-context switches, unstarted/no-group switches, invalid backend updates, recent-message filtering, handoff destination scoping, and runner model normalization.

Impact

This closes the remaining coverage gaps around dynamic backend/model switching without duplicating the existing happy-path assertions from the original implementation PR.

Validation

• npm run typecheck
• npx vitest run src/agent/backend.test.ts src/agent/agentlite-impl.test.ts src/db.test.ts src/agent/backend-handoff.test.ts src/agent/backend-switching.e2e.test.ts src/sdk-types.test.ts src/container-runner.test.ts src/container-agent-backend.test.ts src/task-scheduler.test.ts src/task-lifecycle.test.ts
• npm run build in container/agent-runner
• Changed-file eslint check for the new/modified test files passed with no errors.