Paint selection and whole-line decoration backgrounds across view zones

[Copilot]

View zones (code lens, AI-inserted region headers, etc.) sandwiched between two selected — or whole-line-decorated — lines do not pick up the surrounding background, leaving a visible gap in an otherwise contiguous band:

Changes

• New view part ViewZoneBackgrounds (src/vs/editor/browser/viewParts/viewZoneBackgrounds/)
  • Listens to cursor, decoration, zone, scroll, lines and line-mapping events.
  • On render, walks viewportData.whitespaceViewportData and, for each whitespace whose gap between view lines afterLineNumber and afterLineNumber + 1 is fully enclosed by a selection, emits a cslr selected-text div at the whitespace's vertical offset. Same logic for any isWholeLine decoration with a className, emitting a cdr <className> div — so multi-line backgrounds (e.g. AI-inserted regions) read as one band across the zone.
  • Skips afterLineNumber === 0 (before line 1) and zero-height (hidden-area) whitespaces.
• Wired in view.ts between viewZones.domNode and viewLines.getDomNode(). This z-order covers empty view zones (codelens reservation) while staying under view lines and content widgets, so the codelens text — rendered as a content widget — remains on top.

A gap is considered enclosed by range when:

range.startLineNumber <= beforeLine && range.endLineNumber >= afterLine