Fix: Prevent Connection Pollution by Closing/Resetting Connection on Context Cancellation During BeginTx

[jamboriu]

Resolves #1

Description

This pull request prevents connection pollution in the database pool during transaction initialization (BeginTx).

When a transaction start (START TRANSACTION or setting transaction isolation level) is interrupted by context cancellation, the connection state becomes ambiguous and "dirty" (the server might have actually initiated the transaction, but the driver returns a context error to the client). If this connection is recycled back into the Go database/sql connection pool, subsequent queries will run inside an uncommitted, leaked transaction.

Solution

1. Ambiguity Mitigation: Whenever context cancellation (ctx.Err() != nil) is detected immediately after executing isolation level configurations or START TRANSACTION command roundtrips, the driver explicitly closes the underlying net connection.
2. Pool Eviction: Returns driver.ErrBadConn to the caller. This is the official signal to Go's database/sql connection pool to destroy the connection and evict it from the pool instead of recycling it.
3. Resource Leak Prevention: Closed flag and lock guard ensure clean resource reclamation without race conditions.

Verification & Testing

Implemented comprehensive unit tests in connection_test.go:

• TestBeginTxContextCancellation: Validates that cancellation during transaction start closes the net connection and returns driver.ErrBadConn.
• TestBeginTxContextCancellationDuringIsolationSetting: Validates that cancellation during isolation level writes is handled safely.
• TestBeginTxSuccess: Confirms normal transaction initiation flow when context is not canceled.
• TestBeginTxAlreadyClosedConnection: Confirms error behavior on already closed driver connections.

All tests passed successfully on Go 1.23.