fix(pivot): Update the default row height if content size changed.

[MayaKirova]

Closes #17020

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

Updates Pivot Grid behavior to keep row header/cell alignment correct when the rendered content size changes at runtime (e.g., browser zoom changes), addressing the misalignment described in #17020.

Changes:

• Introduces an overridable onContentSizeChange() hook in IgxGridBaseDirective and routes contentSizeChange handling through it.
• Overrides onContentSizeChange() in IgxPivotGridComponent to recompute the default row height when content size changes.

Reviewed changes

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

File	Description
projects/igniteui-angular/src/lib/grids/pivot-grid/pivot-grid.component.ts	Recomputes pivot default row height on content-size changes to prevent header/cell misalignment after zoom changes.
projects/igniteui-angular/src/lib/grids/grid-base.directive.ts	Refactors contentSizeChange handling into a protected hook to allow derived grids (pivot) to extend the behavior.