Collapse the duplicated send and update-count paths in LibsqlConnection #214

Goal

Collapse the duplicated send and update-count paths in LibsqlConnection. Five places repeat the client call plus LibsqlException to SQLException mapping, and two methods re-implement batch update-count conversion.

Depends on

None. The savepoint pass depends on this one because it deletes executeTransactionStatement.

Pin

LibsqlConnectionTest covers transactions, batches, empty batches, and error mapping. LibsqlSavepointTest covers the savepoint send path.

Scope

• Add sendOnStream(Stmt) and a batch equivalent, each owning the error mapping.
• Make executeBatch(List) one line over steppedUpdateCounts.
• Delete executeTransactionStatement (619 to 626) and point its four call sites at the helper.
• Leave the autoCommit dispatch in executeStatement and executeBatchResults in place.

Preserve

• A failed BEGIN closes the fresh stream and leaves the begin pending.
• A failed COMMIT or ROLLBACK leaves the stream open.
• An empty batch returns an empty list and sends no request.
• Callers still hold stateLock.

Acceptance

• One definition each for send, error mapping, and update-count conversion.
• LibsqlConnection.java is under 750 lines.
• ./mvnw -pl libsql-jdbc -am test green with no assertion edited.
• ./mvnw -pl libsql-itests -am verify green with DOCKER_CONFIG set per AGENTS.md section 4.