fix(grid): fix filering row mask applying on chips

[didimmova]

Closes #17138

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 the quick filter “scroll mask” styling in the grid filtering row so the gradient mask no longer appears/overlaps on hover over the first scroll button (issue #17138).

Changes:

• Moves the filtering scroll “mask” pseudo-elements from the scroll buttons to the filtering-row main container.
• Adds position: relative to the filtering-row main container to correctly anchor the new pseudo-elements.
• Adjusts RTL-specific gradient direction handling for the new mask placement.