fix(core): preserve object pool disposal after overflow

[yordis]

• prevent overflow attempts from leaving the pool in a permanently leased state
• keep disposal callbacks reliable when callers return all leased objects after an overflow

[cursor]

PR Summary

Low Risk
Small, targeted change to ObjectPool lease accounting plus tests; low blast radius but touches shared infrastructure used for readers and scavenge backends.

Overview
Fixes ObjectPool<T>.Get() so a failed lease when the pool is at maxCount no longer leaves shutdown stuck: after rolling back the lease counter, it now invokes OnPoolDisposed() when the count reaches zero while the pool is already marked for disposal.

Adds ObjectPoolTests covering growth to the cap, reuse on Return, and full disposal of all leased items after an overflow Get, whether MarkForDisposal runs before or after returns.

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 7a92f539-0c0f-47e3-80e0-4f81c6df3c29

📥 Commits

Reviewing files that changed from the base of the PR and between a4326df and baa51c9.

📒 Files selected for processing (2)

• src/EventStore.Core.XUnit.Tests/DataStructures/ObjectPoolTests.cs
• src/EventStore.Core/DataStructures/ObjectPool.cs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch yordis/fix-object-pool-overflow-disposal

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