perf: add default configs for assignment V2 (#13577)

## Description

AutoAssignment::RateLimiter#within_limit? returned true early for
inboxes without an AssignmentPolicy, bypassing fair distribution
entirely and allowing unlimited conversation assignment.

## Type of change

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

## Script

- Script to enable users with V2 assignment:
https://www.notion.so/chatwoot/Script-to-migrate-account-to-assignment-V2-30ca5f274c9280f5b8ecfd15e28eeb9c?source=copy_link

## 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

---------

Co-authored-by: Shivam Mishra <scm.mymail@gmail.com>

app/services/auto_assignment/rate_limiter.rb

@@ -2,8 +2,6 @@ class AutoAssignment::RateLimiter
   pattr_initialize [:inbox!, :agent!]
 
   def within_limit?
-    return true unless enabled?
-
     current_count < limit
   end
 
@@ -13,24 +11,18 @@ class AutoAssignment::RateLimiter
   end
 
   def current_count
-    return 0 unless enabled?
-
     pattern = assignment_key_pattern
     Redis::Alfred.keys_count(pattern)
   end
 
   private
 
-  def enabled?
-    config.present? && limit.positive?
-  end
-
   def limit
-    config&.fair_distribution_limit.present? ? config.fair_distribution_limit.to_i : Float::INFINITY
+    config&.fair_distribution_limit.present? ? config.fair_distribution_limit.to_i : 5
   end
 
   def window
-    config&.fair_distribution_window&.to_i || 24.hours.to_i
+    config&.fair_distribution_window&.to_i || 5.minutes.to_i
   end
 
   def config