Add fluent Table API for check constraints

[dereuromark]

Summary

• Add addCheckConstraint() and dropCheckConstraint() methods to the Table class
• Allow managing check constraints with the same fluent API pattern used for indexes and foreign keys
• The adapter already had the underlying implementation - this adds the user-facing Table API

Before (direct adapter access)

public function change(): void
{
    // Awkward - need to get adapter and create objects manually
    $table = $this->table('users');
    $table->addColumn('age', 'integer')->create();
    
    $adapter = $this->getAdapter();
    $constraint = new CheckConstraint('age_check', 'age >= 18');
    $adapter->addCheckConstraint($table->getTable(), $constraint);
}

After (fluent Table API)

public function change(): void
{
    $this->table('users')
        ->addColumn('age', 'integer')
        ->addCheckConstraint('age >= 18', ['name' => 'age_check'])
        ->create();

    // Or with CheckConstraint object
    $constraint = new CheckConstraint('price_positive', 'price > 0');
    $this->table('products')
        ->addCheckConstraint($constraint)
        ->update();

    // Drop a check constraint
    $this->table('users')
        ->dropCheckConstraint('age_check')
        ->update();

    // Check if constraint exists
    if ($this->table('users')->hasCheckConstraint('age_check')) {
        // ...
    }
}

Changes

• New AddCheckConstraint action class
• New DropCheckConstraint action class
• Table::addCheckConstraint() method
• Table::dropCheckConstraint() method
• Table::hasCheckConstraint() method
• Updated Plan.php to handle check constraint actions in gatherConstraints()
• Updated AbstractAdapter::executeActions() to handle the new action types
• Unit tests for the new methods

[markstory]

Looks good to me.

[dereuromark]

Added docs.