Add TransactionScope support

[ratacolita]

Summary

This PR implements System.Transactions.TransactionScope support for AseConnection, addressing feature request #237.

Changes

• New AseEnlistedTransaction class: Implements IEnlistmentNotification to participate in the two-phase commit protocol
• EnlistTransaction(Transaction) method: Added to AseConnection for manual enlistment in System.Transactions
• Auto-enlistment: Connection automatically enlists in Transaction.Current when Open() is called (if ambient transaction exists)
• Enlist connection string parameter: Controls auto-enlistment behavior (default: true)

Usage

// Automatic enlistment (default behavior)
using (var scope = new TransactionScope())
{
    using (var connection = new AseConnection(connectionString))
    {
        connection.Open(); // Auto-enlists in ambient transaction
        // ... execute commands ...
    }
    scope.Complete();
}

// Disable auto-enlistment via connection string
var connectionString = "...;Enlist=false";

// Manual enlistment
connection.EnlistTransaction(Transaction.Current);

Supported Frameworks

• .NET Framework 4.6+
• .NET Standard 2.0
• .NET Core 2.0+

Test Plan

• Test basic TransactionScope commit scenario
• Test TransactionScope rollback (no Complete() called)
• Test explicit EnlistTransaction usage
• Test Enlist=false disables auto-enlistment
• Test error handling during Prepare/Commit/Rollback phases
• Test connection pool behavior with enlisted connections

Fixes #237

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