feat(core): resolve ActiveSync task series master for instance updates

[TDannhauer]

Summary

• Bridge the ActiveSync importer to Nag so that duplicate dead-occurrence task Adds for a recurring series are applied to the series master instead of creating duplicate tasks.

Motivation

EAS clients (iOS Reminders, Outlook) complete a single occurrence of a recurring task by sending a dead-occurrence Add (often reusing a client id) plus a master Modify. Without a way to map such an Add back to the recurring master UID, the importer either ignored the change or created duplicate rows.

Changes

• Horde_Core_ActiveSync_Connector::tasks_resolveSeriesMasterUid() — delegate to the tasks API resolver.
• Horde_Core_ActiveSync_Driver::resolveTaskSeriesMasterUid() — expose the resolver to the ActiveSync importer.

Companion PRs

• horde/activesync: importer uses resolveTaskSeriesMasterUid() and adds master-change detection.
• horde/nag: implements resolveActiveSyncSeriesMasterUid() and the recurrence completion logic.

Test plan

• Existing core test suite unaffected (additive API only).
• Manual device test (iOS 16.1) via the nag/activesync companion changes: complete instance, uncomplete instance, complete whole series, reopen final instance — no duplicate rows.