Add transaction support via sessions - #5

Adds opt-in transaction support on top of the existing single-connection model.

Client::startSession() returns a Session that manages BEGIN, COMMIT, and ROLLBACK on the client's PDO connection. Its API mirrors mongodb/mongodb with startTransaction, commitTransaction, abortTransaction, and withTransaction, allowing existing MongoDB-oriented code to keep working with minimal changes.

withTransaction handles commit and rollback automatically and retries transient failures such as serialization errors and deadlocks a limited number of times. MongoDB's readConcern is mapped to the corresponding PostgreSQL isolation level.

Because all operations already use the single PDO connection owned by the client, an active transaction automatically covers subsequent operations, regardless of whether the session option is explicitly passed. Sessions from a different client are not validated yet; this is documented as a current limitation.

Two related changes are included:

• bulkWrite now joins an existing transaction instead of attempting to open a nested one, which PDO does not support.
• QueryException now exposes the SQLSTATE so the transaction retry logic can identify transient database errors.

Nested transactions are rejected, matching MongoDB's behavior.