fix(SubscriptionManager): replace List with ConcurrentDictionary for abandoned subscriptions (#3617)

[cristipogacean]

Proposed changes

• replace the semaphore-protected List<ISubscription> used for abandoned subscriptions with a readonly ConcurrentDictionary<uint, ISubscription>, eliminating three separate lock acquisitions in SessionClosed, DeleteSubscription, and the publish loop
• add DeleteAbandonedSubscriptionsConcurrentlyAsync integration test to cover the concurrent-delete scenario.

• Fixes [Server] crash during subscription/session teardown in a multi-session telemetry scenario #3617

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[CLAassistant]

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[marcschier]

This needs my fix on the other PR to update the agent image to Windows-2025-vs2026, since msbuild 1.8 is now not found as first in the old image path. Once mine is in, I will take care of resolving merge conflicts and bring this one in. (@romanett @cristipogacean )