Add visual separator to chat terminal button when scrollbar is present

[Copilot]

When the terminal tabs list overflows and displays a scrollbar, the scrollbar ends directly adjacent to the chat terminal button with no visual separation, making it difficult to distinguish the scrollable area from the button.

Changes

• terminalTabsChatEntry.ts: Added overflow detection in update() method

  • Queries for .monaco-scrollable-element and caches reference
  • Compares scrollHeight vs clientHeight to detect scrollbar presence
  • Toggles has-scroll-separator class dynamically

• terminalTabbedView.ts: Pass tabs list container to chat entry via setTabsListContainer()

• terminal.css: Added border-top rule for has-scroll-separator class using --vscode-terminal-border theme variable

The separator appears only when overflow is detected and uses the existing theme border color for consistency.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Chat terminal button reduces space available to actual terminal tabs significantly</issue_title>
<issue_description>Repro:

1. Run ls in chat
2. Create many terminals

Observe:

The scroll area does not extend right up to the chat terminals button:

We should consider some separator above the chat terminals text, maybe only when there's a scroll bar in the main list as they'll be right up next to each other:

This is important since having this chat terminal button is likely going to be a constant for those engaged with chat.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Chat terminal button reduces space available to actual terminal tabs significantly #274319

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