Fix: stop auto-scrolling during selection handle drag - #185206

This PR prevents the text viewport from jumping or "flickering" while a user is dragging selection handles. By disabling automated scrolling during a drag, the viewport remains stable and avoids incorrectly snapping to the bottom of the selection when the user is dragging the top handle.

Fixes #132047

Included a regression test in packages/flutter/test/widgets/editable_text_test.dart to verify that the scroll offset remains stable during a selection drag.

Pre-launch Checklist

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