fix: preserve customer manager ticket visibility for agents

[uttam282005]

Summary

• Restore managed-customer visibility for agents in ticket list queries.
• Keep list-level visibility consistent with has_permission().
• Add a regression test for agents who manage customers whose tickets belong to another team.

Testing

Verified the regression test fails on the original implementation and passes with the fix:

bench --site helpdesk.test run-tests --app helpdesk --module helpdesk.helpdesk.doctype.hd_ticket.test_hd_ticket --test test_customer_managers_see_their_tickets_in_lists

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[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge.

The added list predicate uses the same manager relationship as document permission checks without widening access beyond that existing grant.

_{Reviews (1): Last reviewed commit: "fix: preserve customer manager ticket vi..." | Re-trigger Greptile}