[6.x] Validation improvements & fixes

[riasvdv]

Description

Fixes two validation lifecycle gaps:

• Validatable::prepareForValidation() is now called consistently from Craft ruleset validation, including element rulesets and custom rulesets that override their own preparation hook.

• Plain Validatable classes using Validates without a configured ruleset now fall back to ComponentRules instead of failing when ruleset resolves to false, so their getRules() contract works as expected.

• Also fixes an issue with the legacy yii model serializing rulesets.

Also improves the following

• setAttributes() is now moved from Component to the Validates trait, which makes plain class validation easier
• ComponentRules has been renamed to ValidatableRules to more clearly describe its intent.

Related issues

• [6.x]: Cannot assign array to property craft\base\Model::$ruleset of type CraftCms\RulesetValidation\Ruleset|false #18946