Agents window: Support editor tab display modes in the single-pane layout

[sandy081]

Description

The Agents window single-pane layout currently forces multiple editor tabs and does not integrate correctly with the workbench.editor.showTabs setting.

Support the setting without losing the Agents-specific editor header and Add Tab affordances:

• Respect multiple and single tab modes while the editor area is visible.
• Treat none as single, because the Agents window requires an editor title row.
• Use multiple tabs when only the docked Auxiliary Bar is visible so managed Changes and Files tabs remain directly accessible.
• Keep breadcrumbs in the Agents editor header, without duplicating the path in the single-tab title.
• Keep Add Tab before layout actions, using the standard toolbar trailing separator.
• In single-tab mode, keep all supported Add Tab entries available: Changes, Files, Browser, and Search.
• Preserve normal VS Code editor-window behavior.

Verification steps

1. Enable sessions.layout.singlePaneDetailPanel and reload the Agents window.
2. Set workbench.editor.showTabs to multiple.
   • Verify the Agents side pane displays the normal multi-tab strip.
   • Verify the Add Tab menu is available and missing managed tabs can be reopened.
3. Set workbench.editor.showTabs to single.
   • Verify only the active editor title appears in the top row.
   • Verify the full-width Agents header remains in the row below.
   • Verify breadcrumbs appear only in the header and the path is not repeated beside the editor title.
   • Verify Add Tab appears before the layout actions with the standard toolbar separator after it.
   • Open Add Tab and verify Changes, Files, Browser, and Search are all listed even when their editors are already open.
4. Set workbench.editor.showTabs to none.
   • Verify the Agents window behaves as single-tab mode rather than hiding the title row.
5. Hide the editor area while keeping the docked Auxiliary Bar visible.
   • Verify the title switches to multiple tabs regardless of the configured tab mode.
   • Show the editor area again and verify the configured single/multiple behavior returns.
6. Open a regular VS Code editor window with workbench.editor.showTabs set to single.
   • Verify its existing single-tab title and inline breadcrumbs render normally, with no missing or duplicate header content.