## Description
When an inbox has `enable_auto_assignment` and `assignment_v2` enabled
but no agents are currently online,
`AutoAssignment::AssignmentService#perform_bulk_assignment` still loaded
up to 100 unassigned conversations and iterated each one, calling
`inbox.available_agents` per conversation. Each call hits Redis presence
lookups that return empty, no conversations get assigned, and the loop
finishes having done only wasted work.
For a busy inbox with a long unassigned backlog and offline agents, this
is hundreds of Redis ops per job, multiplied by every
`AutoAssignment::AssignmentJob` enqueue from the per-save handler. The
pressure is significant when inbound volume is high.
This adds a single early-return guard: if
`inbox.available_agents.empty?`, return `0` immediately. Existing
semantics are preserved (jobs are still enqueued on conversation events;
they just exit cheaply when there is no one to assign to).
## Type of change
- [x] Performance improvement (non-breaking change)
## Test coverage
- [x] Added specs
