fix: Enforce team boundaries to prevent cross-team assignments (#13353)

## Description

Fixes a critical bug where conversations assigned to a team could be
auto-assigned to agents outside that team when all team members were at
capacity.

## Type of change

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

## 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

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Changes core assignment selection for both legacy and v2 flows;
misconfiguration of `allow_auto_assign` or team membership could cause
conversations to remain unassigned.
> 
> **Overview**
> Prevents auto-assignment from crossing team boundaries by filtering
eligible agents to the conversation’s `team` members (and requiring
`team.allow_auto_assign`) in both the legacy `AutoAssignmentHandler`
path and the v2 `AutoAssignment::AssignmentService` (including the
Enterprise override).
> 
> Adds test coverage to ensure team-scoped conversations only assign to
team members, and are skipped when team auto-assign is disabled or no
team members are available; also updates the conversations controller
spec setup to include team membership.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
67ed2bda0cd8ffd56c7e0253b86369dead2e6155. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

spec/controllers/api/v1/accounts/conversations_controller_spec.rb

@@ -330,6 +330,7 @@ RSpec.describe 'Conversations API', type: :request do
       context 'when it is an authenticated user who has access to the inbox' do
         before do
           create(:inbox_member, user: agent, inbox: inbox)
+          create(:team_member, user: agent, team: team)
         end
 
         it 'creates a new conversation' do