Fix Agent Session Mode breaks tri-state toggle of chat icon

[Copilot]

In Agent Session Mode, clicking the sparkle chat icon in the title bar did nothing—the tri-state toggle (show → maximize → hide) was broken.

Changes

• Changed chat.agentsControl.clickBehavior from "focus" to "triStateToggle" in resources/workbenchModes/agent-sessions.code-workbench-mode

Root Cause

The Agent Sessions workbench mode overrode the click behavior to "focus", which only focuses the chat input when clicked. When the chat was already visible and focused, this resulted in no visible change.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Agent session mode breaks the tri-state toggle of the chat icon in the title bar</issue_title>
<issue_description>Testing #290241

1. Enable Agent Session Mode
2. Click the sparkle chat icon
   🐛 => nothing happens, even if the chat view is not maximized
   </issue_description>

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

• Fixes Agent session mode breaks the tri-state toggle of the chat icon in the title bar #291099

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