fix: atomically claim conversation to prevent duplicate assignment (#14495)

## Description

Fixes a bug under Assignment V2 where a single conversation could be
reassigned dozens of times in a row by the system, producing long stacks
of "Assigned to X by Automation System via <policy>" activity messages
alternating between agents. After this change each unassigned
conversation is assigned exactly once, even on busy inboxes.

## Fixes # (issue)


## Type of change

- [ ] Bug fix (non-breaking change which fixes an issue)

## How Has This Been Tested?

## How to reproduce
1. Enable `assignment_v2` on an account with at least 2 online agents in
an inbox.
2. Generate sustained resolve/snooze activity in the inbox (each one
enqueues `AutoAssignment::AssignmentJob` for the whole inbox).
3. Watch any one unassigned conversation while the jobs drain — pre-fix
it picks up multiple back-to-back "Assigned to …" activity rows
alternating between agents.


## Checklist:

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

app/models/concerns/auto_assignment_handler.rb

@@ -15,8 +15,10 @@ module AutoAssignmentHandler
     return unless should_run_auto_assignment?
 
     if inbox.auto_assignment_v2_enabled?
-      # Use new assignment system
-      AutoAssignment::AssignmentJob.perform_later(inbox_id: inbox.id)
+      # Coalesces bursts of triggers per inbox. Fine if the job runs even when the
+      # surrounding save rolls back: it only scans the inbox's current unassigned
+      # conversations, so running it for an uncommitted change is harmless.
+      AutoAssignment::AssignmentJob.enqueue_for_inbox(inbox.id)
     else
       # Use legacy assignment system
       # If conversation has a team, only consider team members for assignment