Fix unsynchronized read of transaction in ExecuteOnConnection

[Copilot]

ExecuteOnConnection used a double-checked locking pattern where the initial if (this.transaction?.Connection != null) guard read this.transaction outside transactionLock. Since BeginTransaction/CommitTransaction write under the lock, this was a data race — a stale null read could cause operations to bypass an active transaction and open a new connection instead.

Changes

• SqliteDataSource.ExecuteOnConnection: Removed the outer unsynchronized check. The lock now guards the first read of this.transaction; the captured value is used to branch.

// Before: unsynchronized outer read, then re-check inside lock
if (this.transaction?.Connection != null)       // ← data race
{
    lock (transactionLock)
    {
        var currentTransaction = this.transaction;
        if (currentTransaction?.Connection != null)
            return func(currentTransaction.Connection, currentTransaction);
    }
}

// After: single read, always under the lock
lock (transactionLock)
{
    var currentTransaction = this.transaction;
    if (currentTransaction?.Connection != null)
        return func(currentTransaction.Connection, currentTransaction);
}

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