fix(modal): adjust scroll to account for gradient

[warrenmblood]

Closes #19277

On a blur event when Modal has scrollable content, adjust scroll if needed to prevent element in focus from being obscured by the gradient styling.

Made changes to the following components:

• React:
  ComposedModal
Modal

• Web Component
  Modal

Changelog

New

• In onBlur handler, calculate spacing needed and update scrollTop of Modal content

Testing / Reviewing

• CI tests should pass
• Go to Storybook and check that gradient styling does not obscure the focused element while navigating only using TAB through the scrollable Modal. Check the following:
  React:
  Modal w/ AI Label
  ComposedModal w/ AI Label
  ComposedModal w/ Scrolling Content
  Web Component:
  Modal w/ AI Label

PR Checklist

As the author of this PR, before marking ready for review, confirm you:

• Reviewed every line of the diff
• Updated documentation and storybook examples
• Wrote passing tests that cover this change
• Addressed any impact on accessibility (a11y)
• Tested for cross-browser consistency
• Validated that this code is ready for review and status checks should pass

More details can be found in the pull request guide

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[codecov]

Codecov Report

Attention: Patch coverage is 47.36842% with 20 lines in your changes missing coverage. Please review.

Project coverage is 84.47%. Comparing base (4c395af) to head (ce1d35f).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
...act/src/components/ComposedModal/ComposedModal.tsx	54.54%	10 Missing ⚠️
packages/react/src/components/Modal/Modal.tsx	37.50%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19534      +/-   ##
==========================================
- Coverage   84.56%   84.47%   -0.10%     
==========================================
  Files         373      373              
  Lines       14662    14695      +33     
  Branches     4848     4816      -32     
==========================================
+ Hits        12399    12413      +14     
- Misses       2116     2135      +19     
  Partials      147      147              

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