feat(kit/forms): support Ionic Signal Forms error text

[rdlabo]

Overview

Adds Angular Signal Forms integration to @rdlabo/ionic-angular-kit as the new @rdlabo/ionic-angular-kit/forms secondary entry point.

This centralizes the Ionic-specific adapter and state-class configuration that applications previously had to implement themselves. Validation messages and localization remain owned by each application.

Changes

• Add KitIonicFormField, a sibling adapter for Angular's standard FormField
• Project the first non-empty string validation message to Ionic's errorText
• Support ion-input, ion-textarea, ion-select, ion-checkbox, ion-radio-group, and ion-toggle
• Preserve explicit errorText and [errorText] bindings as application overrides
• Add provideKitIonicSignalForms() for Angular and Ionic form-state classes
• Document standalone component and application-provider setup
• Verify the secondary entry point through the packed-package consumer test

Compatibility

• No breaking changes to existing entry points
• The new forms entry point targets Angular 22 Signal Forms
• Native controls continue to use Angular's FormField directly

Validation

• npm run prebuild:kit
• npx ng test kit --watch=false — 952 tests passed
• npm run lint
• npm run test:package-consumer