Description

This change adds a scrollPadding property to DropdownMenu, forwarding it to the internal TextField widget.

DropdownMenu wraps a TextField internally but did not expose the scrollPadding parameter. Both TextField and SearchBar already provide this property, so DropdownMenu should forward it for consistency. The scrollPadding property controls the padding around the text field when it scrolls into view (e.g., when the keyboard appears), allowing developers to ensure the field remains visible with appropriate spacing.

The default value is EdgeInsets.all(20.0), matching TextField's default to preserve existing behavior.

This PR implements the analysis from my earlier comment on this issue.

Related Issues

Fixes #182890

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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