Resize cell editor width when resizing window horizontally

[DonJayamanne]

Fixes #273603

[Copilot]

Pull Request Overview

This PR fixes an issue where the cell editor width wasn't properly updating when the window was resized horizontally. The changes introduce a type-safe CellLayoutChangeReason type to track why layout changes occur, and update the editor width logic to use the viewCell's layout info during horizontal resize events.

Key changes:

• Introduces CellLayoutChangeReason type for type-safe layout change tracking
• Updates editor width calculation to use viewCell.layoutInfo.editorWidth for nbLayoutChange and viewCellLayoutChange events
• Replaces string literals with typed constants throughout the codebase

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/vs/workbench/contrib/notebook/browser/view/cellParts/codeCell.ts	Adds CellLayoutChangeReason type, updates method signatures to use the new type, and modifies editor width calculation logic to properly handle horizontal resizing
src/vs/workbench/contrib/notebook/test/browser/view/cellPart.test.ts	Updates test code to use typed layout change reasons instead of string literals