Add descriptive error messages for composite unique constraints

[dereuromark]

Summary

• Adds descriptive error messages for composite unique constraints in generated buildRules() methods
• Messages are wrapped in __() for i18n support
• Only applied to multi-column unique constraints (single-column constraints unchanged)

Before

$rules->add($rules->isUnique(['field_1', 'field_2']), ['errorField' => 'field_1']);

After

$rules->add($rules->isUnique(['field_1', 'field_2']), ['errorField' => 'field_1', 'message' => __('This combination of field_1 and field_2 already exists')]);

Additional fixes included

• Fix schema.php: use int for length fields instead of strings (was causing bootstrap errors)
• Fix EntryCommandTest to work with both old and new CakePHP help output formats
• Fix testGetEntityPropertySchema to handle PostgreSQL returning timestampfractional instead of timestamp
• Add plugin classes for all test plugins to fix CakePHP 5.3.0 deprecation warnings
• Update composer.json autoload-dev to include all test plugin namespaces
• Add ignore rules for generic type PHPStan errors from CakePHP 5.3.0
• Fix writeFile method name casing in ModelCommand (was writefile)

Test plan

• All existing model bake tests pass
• Tests verify composite unique constraints get descriptive messages
• Single-column unique constraints remain unaffected
• Works on MySQL, PostgreSQL, and SQLite
• Works with both CakePHP "lowest" and "highest" dependency versions