Ensure scoped table retrieval for MySQL/MariaDB connections

[zaleslaw]

Closes #1746

Fix #1746: readAllSqlTables leaks tables from other databases on MySQL/MariaDB

1. Problem

readAllSqlTables and DataFrameSchema.readAllSqlTables called without an explicit catalogue
incorrectly return tables from all accessible databases when connected to MySQL or MariaDB.

Root cause: both functions passed null directly to DatabaseMetaData.getTables(null, ...).
On MySQL/MariaDB, null means all catalogs, so a connection scoped to
jdbc:mysql://host/mydb would enumerate tables from every database visible to that user.
Subsequent SELECT * FROM table_from_another_db then fails with "table doesn't exist"
because the query runs in the wrong database context.

This is a MySQL/MariaDB-specific behavior: PostgreSQL and SQLite connections are inherently
single-database, so getTables(null, ...) there already returns only the current DB.

2. Solution

When catalogue is not provided by the caller, fall back to connection.catalog
(standard JDBC API — returns the database name from the JDBC URL).
The same pattern is already used in DbType.getTableColumnsMetadata.

val effectiveCatalogue = catalogue ?: try {
    connection.catalog.takeUnless { it.isNullOrBlank() }
} catch (_: Exception) { null }

If the URL has no database (jdbc:mysql://host without /dbname), connection.catalog
returns null/"", which falls through to null — preserving the existing
"all catalogs" behavior for that case.

Fixed in two places:

• readJdbc.kt — DataFrame.readAllSqlTables(connection, ...)
• readDataFrameSchema.kt — DataFrameSchema.readAllSqlTables(connection, ...) (same bug, hardcoded null)

3. Situation by Database

Database	Affected	Reason
MySQL	Fixed	getTables(null,...) returns all databases; connection.catalog = DB from URL
MariaDB	Fixed	Same behavior as MySQL
SQL Server	Fixed	Same catalog/database model; same fix applies
PostgreSQL	No change	Cross-database access impossible; getTables already scoped to current DB
SQLite	No change	Single-file = single catalog; connection.catalog returns null
H2	No change	In-process, single DB per connection; not affected
DuckDB	No change	connection.catalog returns null for default in-memory catalog

Tests added (all @Ignore, require local server) in:

• local/mysqlTest.kt
• local/mariadbTest.kt
• local/mssqlTest.kt

Each test creates a second database with a unique table, connects to DB1 by URL,
calls readAllSqlTables without catalogue, and asserts the second DB's table is absent.

[Jolanrensen]

if you're not interested in the exception, maybe you could try runCatching { ... }.getOrNull(). Just an alternative way to write it, but both are valid of course :)

[zaleslaw]

Agree, it could be better for reading or handling