fix: proactive FSI revert on transaction rollback

[Copilot]

• Read issue InsertBulkAsync fails with "Not enough space" due to an unrecoverable desync between in-memory FSI and physical page (v4.3) #58 comments and PR fix: correct FSI before throwing "Not enough space" to break poisoned-cache loop #59 (reactive fix already merged)
• Add SnapshotForTransaction/RollbackTransaction/CommitTransaction to FreeSpaceIndex
• Add OnCommit event to Transaction (concrete class only, not ITransaction)
• In DocumentCollection.DeleteCore, snapshot FSI before modification and subscribe once per transaction to restore/cleanup
• Update regression test to assert no exception on first insert attempt (proactive fix)
• Address review feedback:
  • Remove OnCommit from public ITransaction (breaking change); keep only on concrete Transaction
  • Cast to Transaction in DeleteCore before subscribing to OnCommit
  • Invoke OnCommit and OnRollback handlers best-effort (GetInvocationList + per-handler try/catch)
• Build and test: 2005 pass, 6 pre-existing skips, 0 failures

[Copilot]

Pull request overview

This PR fixes a rollback-induced Free Space Index (FSI) desynchronization by snapshotting per-transaction FSI values before compaction updates and restoring them during transaction rollback, so inserts after a rollback don’t hit stale “optimistic” free-space entries.

Changes:

• Add per-transaction FSI snapshot/restore APIs (SnapshotForTransaction, RollbackTransaction, CommitTransaction) to proactively revert in-memory FSI state on rollback.
• Introduce an OnCommit transaction lifecycle event and wire FSI snapshot cleanup to OnCommit/OnRollback from DocumentCollection.DeleteCore.
• Update the regression test to require the first insert after rollback to succeed without a transient exception.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/BLite.Tests/InsertBulkTests.cs	Updates the rollback/FSI regression test to assert insert succeeds on first attempt.
src/BLite.Core/Transactions/Transaction.cs	Adds OnCommit event invocation after commit/CDC publication.
src/BLite.Core/Transactions/ITransaction.cs	Extends the public transaction interface with OnCommit.
src/BLite.Core/Collections/FreeSpaceIndex.cs	Adds transaction snapshot tracking and rollback/commit replay/cleanup.
src/BLite.Core/Collections/DocumentCollection.cs	Snapshots FSI before delete compaction updates and subscribes to transaction lifecycle events once per txn.

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[mrdevrobot]

@copilot apply changes based on the comments in this thread

[Copilot]

@copilot apply changes based on the comments in this thread

Done in d5f335a:

• OnCommit removed from ITransaction — kept only on the concrete Transaction class to avoid a breaking change to the public interface.
• DocumentCollection.DeleteCore casts to Transaction before subscribing to OnCommit.
• Both OnCommit and OnRollback are now invoked best-effort via GetInvocationList() with a per-handler try/catch, so a failing handler neither surfaces as a commit/rollback failure nor prevents subsequent handlers from running.

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