Model picker: no visual contrast between selected/unselected context-window value (e.g. "264K 1M")

Describe the bug

In the copilot CLI's model picker (the panel shown when switching models / after a Model changed from ... for this session message), the "Context" column for a model that offers two context-window sizes shows both values together, e.g. 264K 1M for Claude Opus 5.

There is no visible difference (weight, color, dimming, checkmark, etc.) between the currently-selected context-window value and the other one. Looking at the row, it's impossible to tell whether the session is currently using the 264K or the 1M context window — you have to trust the small Model changed from ... (1M context) for this session log line above instead of the picker itself.

This is not a light-vs-dark theme color problem: I compared VS Code's light theme and dark theme (both using the integrated terminal), and the row renders identically in both — same lack of contrast between the two values in either theme. Screenshots attached (light theme, then dark theme).

Affected version

GitHub Copilot CLI 1.0.85

Steps to reproduce the behavior

1. Run copilot inside VS Code's integrated terminal.
2. Trigger the model picker (e.g. run /model, or open the panel shown after switching models for the session).
3. Look at the "Context" column for a model with two context sizes, e.g. "Claude Opus 5" showing 264K 1M.
4. Try to tell, from the row's styling alone, which of the two values (264K or 1M) is the one currently applied to the session.
5. Toggle it (Tab) and look again — the rendering doesn't change in a way that clarifies which value is now active.

Expected behavior

The currently active context-window value should be clearly distinguishable from the inactive one (e.g. bold + accent color for the selected one, dimmed/greyed for the other), in both light and dark terminal themes, so the user can tell at a glance which one is selected without having to scroll up and re-read the session log.

Additional context

• Operating system: macOS (Darwin)
• Terminal: VS Code integrated terminal
• Reproduced identically in VS Code light theme and dark theme — same lack of contrast between the two Context values in either theme:

VS Code light theme:

VS Code dark theme:

• Minor but real usability impact: before noticing the small log line above the picker, I genuinely couldn't tell whether my context-window selection had taken effect at all.

從 copilot CLI 的 /model 入口開始，重現模型選擇器，並使用一個顯示兩個內容值（例如 264K 和 1M）的模型。檢查選擇器的渲染，並使用 Tab 切換值；當作用中的值在淺色和深色終端主題中都能與非作用中的值形成明顯的視覺區別時，即可完成。