fix(persistent-subscriptions): preserve parked message accounting

[yordis]

• Failed park writes must not erase the last confirmed parked event or make operator statistics report fewer parked messages than actually exist.

[cursor]

PR Summary

Medium Risk
Touches persistent subscription parking state used for operator metrics and retry behavior; the change is small and guarded by a focused test, but incorrect accounting could mislead ops during partial write failures.

Overview
Park write failures no longer corrupt in-memory parked-message stats. WriteStateCompleted in PersistentSubscriptionMessageParker now updates _lastParkedEventNumber and _oldestParkedMessage only when the write returns OperationResult.Success, instead of doing so on every completion (including timeouts and other failures).

That keeps ParkedMessageCount and GetOldestParkedMessage aligned with what was actually committed to the parked stream when a later park attempt fails. A new test parks one message successfully, then simulates a failed second park (CommitTimeout) and asserts the failure is surfaced while count and oldest timestamp stay unchanged.

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