fix(mysql): do not share the SQL-editor socket with Table Browser #802

Summary

Pool key is id::database::readWrite. QueryaSwitchingBody keeps home (SQL) mounted under the table. When the saved databaseName matches the tree database, both acquire the same MySQLConnection.

SQL toolbar START TRANSACTION (and leftover SET / USE) then collide with Table Browser START TRANSACTION / COUNT(*). No open-tx warning (Postgres has one on Overview↔SQL; SQLite has #794).

SQL pool database is connectionRow.databaseName ?? '' (empty key if unset); table uses the tree database — they share only when those strings match (the common case).

Scope

• Separate pool keys, or drop the SQL lease while a table is focused.
• Do not START TRANSACTION on a session already in a transaction.
• Warn or refuse opening a table while the SQL tab has an open transaction on that database.

Out of scope

• Postgres fix(postgres): do not share the SQL-editor read-write socket with Table Browser #785 / SQLite fix(sqlite): do not share the SQL-editor Database with Table Browser #794 (same class; MySQL-specific apply path).