fix(combo/simple-combo): prevent Escape from closing parent container on clear - 21.1.x

[georgianastasov]

Closes #17119

Additional information (check all that apply):

• Bug fix
• New functionality
• Documentation
• Demos
• CI/CD

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

[Copilot]

Pull request overview

Fixes an issue where pressing Escape in igx-combo / igx-simple-combo inside a parent container (e.g., igx-dialog) can propagate the key event and cause the parent to close while the combo is clearing/closing.

Changes:

• Stop Escape keydown propagation in IgxComboComponent.onEscape.
• Stop Escape keydown propagation in IgxSimpleComboComponent.handleKeyDown.
• Add unit tests asserting stopPropagation() is invoked for Escape in relevant states.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
projects/igniteui-angular/simple-combo/src/simple-combo/simple-combo.component.ts	Stops Escape keydown propagation during simple-combo keyboard handling.
projects/igniteui-angular/simple-combo/src/simple-combo/simple-combo.component.spec.ts	Adds specs asserting Escape keydown propagation is stopped.
projects/igniteui-angular/combo/src/combo/combo.component.ts	Stops Escape keydown propagation in combo host listener handler.
projects/igniteui-angular/combo/src/combo/combo.component.spec.ts	Adds specs asserting Escape keydown propagation is stopped.