fix: reject unsupported database drivers in make:migration --session - #10546

Description

Discovered on #10545 . A smoke run of --session against a SQLite3 group produced a migration that cannot run, so this fixes the underlying bug on develop first.

make:migration --session renders the session table columns only for MySQLi and Postgre. For any other driver the template falls through and the command silently writes a migration that indexes a timestamp column it never defined:

$this->forge->addField([
    'id' => ['type' => 'VARCHAR', 'constraint' => 128, 'null' => false],
]);
$this->forge->addKey('id', true);
$this->forge->addKey('timestamp');
$this->forge->createTable('ci_sessions', true);

The user only finds out when spark migrate fails. The per-driver split is deliberate (8600032): DatabaseHandler writes timestamp with raw now(), so the column must be a real timestamp type, and the user guide already limits database sessions to MySQL and PostgreSQL. What was missing is a refusal for everything else.

The command now checks the group's DBDriver before generating and returns EXIT_ERROR with a message naming the group and driver. An undefined group is reported too instead of raising an undefined-property warning.

$ php spark make:migration --session --dbgroup tests
Database sessions are only supported on MySQLi and Postgre. The "tests" database group uses the "SQLite3" driver.

$ php spark make:migration --session --dbgroup bogus
The "bogus" database group is not defined.

New language strings: CLI.generator.undefinedDatabaseGroup and CLI.generator.unsupportedSessionDriver.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide