Fix editor sticky scroll shadow missing

[yogeshwaran-c]

Summary

Fixes #300355

The editor sticky scroll's bottom shadow was not rendering because the outward box-shadow on the widget root was clipped by its own overflow: hidden.

This restructures the sticky scroll widget DOM to match the proven pattern used by the tree view's sticky scroll:

• sticky-widget-content (new inner wrapper): carries overflow: hidden and the bottom border, containing the line numbers and line content
• sticky-widget-shadow (new element): positioned just below the content area with an inset box-shadow that renders reliably regardless of parent overflow
• sticky-widget (root): no longer has overflow: hidden, allowing the shadow element to extend beyond the content bounds

Test plan

• Open a file with enough nested scopes to trigger sticky scroll (e.g., nested functions/classes)
• Scroll down so sticky scroll headers appear at the top of the editor
• Verify a visible shadow appears at the bottom edge of the sticky scroll, separating it from the editor content below
• Test with Dark Modern, Light Modern, and High Contrast themes
• Verify the sticky scroll in the Explorer tree view still has its shadow (unchanged)
• Verify sticky scroll line content is still clipped horizontally (no overflow)
• Verify folding icons, hover effects, and click-to-navigate still work on sticky scroll lines
• Verify sticky scroll in peek view editors still renders correctly