fix: scrollbar select widgets style (#41656)

## Description
> [!TIP]  
> _Add a TL;DR when the description is longer than 500 words or
extremely technical (helps the content, marketing, and DevRel team)._
>
> _Please also include relevant motivation and context. List any
dependencies that are required for this change. Add links to Notion,
Figma or any other documents that might be relevant to the PR._

Dropdown lists in Select, Multi-select, and Tree Select widgets now use
clearer, more consistent scrollbars. We fixed cases where list content
could show through the scrollbar area, aligned scrollbar styling across
these widgets, and tuned appearance (thickness, color, spacing) so long
option lists look cleaner and more in line with the rest of the product.
Existing apps get the updated look automatically; no migration or new
settings are required.

Fixes #`Issue Number`  
_or_  
Fixes https://github.com/appsmithorg/appsmith-ee/issues/8816
> [!WARNING]  
> _If no issue exists, please create an issue first, and check with the
maintainers if the issue is valid._

## Automation

/ok-to-test tags="@tag.Visual"

### :mag: Cypress test results
<!-- This is an auto-generated comment: Cypress test results  -->
> [!TIP]
> 🟢 🟢 🟢 All cypress tests have passed! 🎉 🎉 🎉
> Workflow run:
<https://github.com/appsmithorg/appsmith/actions/runs/23917434698>
> Commit: b686ac63916178e65a319986da3a3c1d7dc35d34
> <a
href="https://internal.appsmith.com/app/cypress-dashboard/rundetails-65890b3c81d7400d08fa9ee5?branch=master&workflowId=23917434698&attempt=1"
target="_blank">Cypress dashboard</a>.
> Tags: `@tag.Visual`
> Spec:
> <hr>Thu, 02 Apr 2026 19:34:52 UTC
<!-- end of auto-generated comment: Cypress test results  -->


## Communication
Should the DevRel and Marketing teams inform users about this change?
- [ ] Yes
- [ ] No


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Style**
* Standardized dropdown list item height across select and tree-select
widgets for consistent spacing.
* Applied uniform dropdown sizing to single-, multi- and tree-select
controls for predictable behavior.
* Improved and unified virtual-list scrollbar styling in dropdown menus
for better visual consistency and usability.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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Co-authored-by: Stacey Levine <stacey@levineclan.net>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>