Markdown editor: Compact Find hides Match Case, Whole Word, and Regex options

## Description

In the experimental Markdown editor, the Find UI removes all search-option controls at compact desktop widths. At approximately 500 CSS px, invoking Cmd/Ctrl+F shows the query and navigation controls, but **Match Case**, **Whole Word**, and **Use Regular Expression** are no longer discoverable or usable. This prevents users from refining a search whenever the editor is narrow, including split-editor and small-window layouts.

## Reproduction Steps

1. Open the experimental Markdown editor on desktop at a wide editor width.
2. Invoke Find with Cmd+F (macOS) or Ctrl+F (Windows/Linux).
3. Confirm that Match Case, Whole Word, and Use Regular Expression are available.
4. Resize the editor/window to approximately 500 CSS px wide.
5. Invoke Find again.

## Expected

The search options remain discoverable and keyboard/mouse usable at compact widths. The Find chrome should adapt, for example with a two-row layout and by spanning the available right rail, rather than removing functionality.

## Actual

Match Case, Whole Word, and Use Regular Expression vanish from the Find UI at compact widths.

## Confirmed Root Cause

[`packages/core/src/editorView.ts` around lines 245-248](https://github.com/microsoft/vscode-packages/blob/main/packages/core/src/editorView.ts#L245-L248) enables compact mode when usable content width is at most 400 px. Because the layout reserves 48 px of left padding and 86 px of right padding, this threshold is reached at an editor width of roughly 534 px.

In compact mode, [`packages/core/src/styles/find.css` around lines 310-312](https://github.com/microsoft/vscode-packages/blob/main/packages/core/src/styles/find.css#L310-L312) explicitly applies `display: none` to `.md-find-options`, removing the three option controls rather than reflowing them.

## Additional Context

A responsive two-row layout would preserve the options without requiring the single-row Find UI to fit within the reduced content column. Allowing the Find chrome to use the right rail would also recover some of the currently reserved width.