Fix IllegalArgumentException during E4/RCP application shutdown

[Copilot]

Fixes eclipse-platform#3509. During application shutdown, context deactivation triggers part activation logic that attempts to set a window as the selected element after its toBeRendered flag has been set to false, causing ElementContainerImpl.setSelectedElement() to throw IllegalArgumentException: The selected element must be visible in the UI presentation.

Changes

Added isToBeRendered() guards before setSelectedElement() calls in shutdown-sensitive code paths:

• PartServiceImpl: activate(), switchPerspective(), createElement(), internalHidePart()
• ModelServiceImpl: bringToTop(), showElementInWindow()
• ApplicationPartServiceImpl: switchPerspectiveInternal()
• CommandLineOptionModelProcessor: process()

Example pattern applied:

// Before
window.getParent().setSelectedElement(window);

// After
if (window.isToBeRendered()) {
    window.getParent().setSelectedElement(window);
}

This pattern already existed in the codebase (line 1407 of PartServiceImpl) but wasn't consistently applied across all setSelectedElement() call sites.

Testing

Added EPartServiceTest.testActivatePartDuringWindowCleanup() to verify activation logic works correctly when window is marked for removal.

Original prompt

Analyze and propose a fix for eclipse-platform#3509

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