Route outline navigation to the source editor's group across windows

[Copilot]

When the Outline view (hosted in the main window) tracks an editor in an auxiliary window, clicking a symbol sometimes activates the main window instead of navigating within the auxiliary window's editor.

DocumentSymbolsOutline.reveal calls ICodeEditorService.openCodeEditor(input, source, sideBySide), where source is the auxiliary window's editor. The workbench implementation then opens via editorService.openEditor(input, ACTIVE_GROUP). Focusing/clicking the outline can shift the most-recently-active group to a main-window group, so ACTIVE_GROUP no longer points at the source's group and navigation lands in the wrong window.

Changes

• CodeEditorService.doOpenCodeEditor: when source is provided and sideBySide is false, resolve the visible pane whose code editor equals source and target its group explicitly; fall back to ACTIVE_GROUP only if the source is no longer visible.
• Reused the same visibleEditorPanes lookup that already drove the preview-pinning logic, so the resolved group is shared between the pin step and the open step.

let sourceGroup: IEditorGroup | undefined;
if (source && !sideBySide) {
    for (const visiblePane of this.editorService.visibleEditorPanes) {
        if (getCodeEditor(visiblePane.getControl()) === source) {
            sourceGroup = visiblePane.group;
            break;
        }
    }
}
// ...
const control = await this.editorService.openEditor(
    input,
    sideBySide ? SIDE_GROUP : (sourceGroup ?? ACTIVE_GROUP)
);

Behavior is unchanged when the source's group already is the active group, when source is null, or when opening side-by-side.