feat(grid): style inputs in the context of cell

[didimmova]

Closes #16360

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

This PR adds styling overrides for input components within grid cells to ensure proper visual integration. The changes apply to various Ignite UI input components (input-group, combo, select, date/time pickers) to make them display correctly within the grid cell context.

Key changes:

• Extends cell input overrides placeholder
• Sets height to 'auto' for multiple input component types within cells
• Applies transparent background to igx-input-group components

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[desig9stein]

Since you are extending this here, you can remove that same extend from igx-grid-td--editing since the base class igx-grid_td is always present

[desig9stein]

I try to create a global theme for the simple combo, and I noticed this visual behavior.

Use this theme to test:

igx-simple-combo {
    @include css-vars-from-theme(
        $theme: combo-theme(
            $schema: $schema,
            $toggle-button-background: red,
            $toggle-button-foreground: green,
        ),
        $name: 'ig-combo',
    );
}

There are some rules with an important flag on them; we have to handle this in the overrides

[didimmova]

@simeonoff @sdimchevski I fixed the disabled states for the combo and input in the context of grid cell to use the cell-disabled-foreground.

But I have a question - for the idle state of the toggle button, should it be using it's own toggle-button-foreground variable, or it's better to use the foreground from the cell?