fix(forms): gate invalid state on touched or dirty

[viktorkombov]

Closes #17671

Description

With [formField], Angular writes the field's raw invalid() into the invalid input of igx-checkbox, igx-switch and igx-radio-group whenever it changes, bypassing the touched-or-dirty rule these controls apply for reactive and template-driven forms — e.g. a pristine field painted the error style right after reset() or when a cross-field rule made it required. Same ordering issue #17637 fixed for required.

• invalid is now a gated setter on CheckboxBaseDirective and IgxRadioGroupDirective: with a bound form control, true is coerced to false while the control is untouched and pristine (checkbox/switch/radio became delegating accessors to keep @Input/@HostBinding).
• The radio group derives invalid from the control on status changes instead of always clearing it, since Signal Forms only push validity on change.
• Fixed the group's registerOnTouched never reaching the radio buttons (the forms directive registers before any buttons exist), so blur now marks the field touched.

Tests: 3 new regression specs (fail without the fix); full non-grid suite green (3386), lint clean. Changelog updated; no API changes.

🤖 Generated with Claude Code

Type of Change (check all that apply):

• Bug fix
• New functionality
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactoring (no functional changes)
• Documentation
• Demos
• CI/CD
• Tests
• Changelog
• Skills/Agents

Component(s) / Area(s) Affected:

How Has This Been Tested?

• Unit tests
• Manual testing
• Automated e2e tests

Test Configuration:

• Angular version:
• Browser(s):
• OS:

Screenshots / Recordings

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines)
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes (migrations guidelines)
• This PR includes behavioral changes and the feature specification has been updated with them
• Accessibility (ARIA, keyboard navigation, focus management) has been verified

[Copilot]

Copilot review overview

🟢 Approval recommended

The focused implementation consistently addresses the reported ordering issue and includes targeted regression coverage.

Review effort: Balanced
Findings: None

What changed in this PR

This PR aligns Signal Forms invalid styling with reactive/template-driven forms by gating invalid state until controls are touched or dirty.

Changes:

• Gates checkbox, switch, radio, and radio-group invalid states.
• Correctly propagates radio-group invalid and touched states.
• Adds regression tests, documentation, and changelog coverage.

File	Description
CHANGELOG.md	Documents the Signal Forms fix.
skills/​igniteui-angular-components/​references/​form-controls.md	Clarifies invalid-state behavior.
projects/​igniteui-angular/​directives/​src/​directives/​checkbox/​checkbox-base.directive.ts	Centralizes invalid-state gating.
projects/​igniteui-angular/​checkbox/​src/​checkbox/​checkbox.component.ts	Delegates invalid state to the base directive.
projects/​igniteui-angular/​checkbox/​src/​checkbox/​checkbox.component.spec.ts	Tests reset behavior.
projects/​igniteui-angular/​switch/​src/​switch/​switch.component.ts	Delegates invalid state to the base directive.
projects/​igniteui-angular/​switch/​src/​switch/​switch.component.spec.ts	Tests conditional validation.
projects/​igniteui-angular/​radio/​src/​radio/​radio.component.ts	Supports group-driven invalid propagation.
projects/​igniteui-angular/​radio/​src/​radio/​radio-group/​radio-group.directive.ts	Gates and propagates group invalid state.
projects/​igniteui-angular/​radio/​src/​radio/​radio-group/​radio-group.directive.spec.ts	Tests reset behavior.

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