refactor(connectors): share MySQL/MariaDB schema introspection (#468)

The MySQL and MariaDB connectors carried near-identical copies of every
INFORMATION_SCHEMA / SHOW CREATE method (getSchemas, getTables, getViews,
tableExists, getTableIndexes, getTableSchema, getTableComment,
getStoredProcedures, getStoredProcedureDetail, getDefaultSchema), differing
only in the driver's result shape. Fixes landed in one and not the other.

Move those into src/connectors/mysql-family-introspection.ts as functions
over a row-array query adapter, following the convention already used by
mysql-family-health-check.ts. Each connector keeps a small requireQuery()
that owns the "not connected" check and unwraps its driver's result shape
(mysql2 resolves to [rows, fields]; mariadb to rows). The driver-specific
parts (DSN parsing, pooling, timeouts, executeSQL) stay in each index.ts.

No behaviour change: the SQL, parameters, error messages and logging are
unchanged, and the adapter calls pool.query with the same arity as before.


Claude-Session: https://claude.ai/code/session_01T79tgGfqWNqUeTwhRc65MJ

Co-authored-by: Claude <noreply@anthropic.com>

Author: tianzhou