refactor(*): unused standalone imports cleanup

[damyanpetev]

Since v19 Angular reports unused standalone imports. Those are surfaced by the lang service if you land on the file, but the lib build itself doesn't log them. Those surfaced from the strict checks from the demos app instead, for example:

[WARNING] TS-998113: IgxButtonDirective is not used within the template of IgxActionStripComponent [plugin angular-compiler]

    projects/igniteui-angular/src/lib/action-strip/action-strip.component.ts:80:8:
      80 │         IgxButtonDirective,
         ╵  

https://github.com/IgniteUI/igniteui-angular/actions/runs/12672621639/job/35317053267#step:16:32

Now that those are all cleared up, also upgrading all of the Extended Diagnostics to errors to prevent future such leftovers.

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