IGNITE-28106 .NET: Fix tx rollback to ignore connection errors

[ptupitsyn]

https://issues.apache.org/jira/browse/IGNITE-28106

[Copilot]

Pull request overview

This PR fixes the .NET thin client's transaction rollback to gracefully handle connection errors instead of throwing exceptions. When the client connection is closed before a rollback is attempted, the server will automatically roll back the transaction (due to session timeout), so the client-side IgniteClientConnectionException can be safely ignored.

Changes:

• RollbackAsync() in LazyTransaction.cs now catches and ignores IgniteClientConnectionException, since a connection loss means the server already rolled back the transaction.
• Added a test verifying that calling RollbackAsync() on a transaction after the client connection is disposed does not throw.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
modules/platforms/dotnet/Apache.Ignite/Internal/Transactions/LazyTransaction.cs	Wraps rollback logic in try-catch to ignore IgniteClientConnectionException, with guard clause refactoring
modules/platforms/dotnet/Apache.Ignite.Tests/Transactions/TransactionsTests.cs	Adds integration test verifying rollback on a closed connection does not throw

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