Fix Editor TextChanged event trimmed in iOS release builds (.NET 10)

[Copilot]

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Description of Change

The .NET 10 trimmer removes MauiTextView.OnChanged event handler during iOS release builds, breaking Editor's TextChanged event. The linker's static analysis doesn't recognize the indirect event subscription through UITextView.Changed and incorrectly marks the handler as unused.

Fix: Add [UnconditionalSuppressMessage("Memory", "MEM0003")] to preserve the event handler, following the pattern established in CarouselViewController2.cs, ItemsViewController.cs, and MauiSearchBar.cs.

[UnconditionalSuppressMessage("Memory", "MEM0003", Justification = "Event handler for UITextView.Changed - proven safe in test: MemoryTests.HandlerDoesNotLeak")]
void OnChanged(object? sender, EventArgs e)
{
    HidePlaceholderIfTextIsPresent(Text);
    TextSetOrChanged?.Invoke(this, EventArgs.Empty);
}

Changes:

• src/Core/src/Platform/iOS/MauiTextView.cs: Added attribute to preserve OnChanged handler
• src/Controls/tests/TestCases.HostApp/Issues/EditorTextChangedIOS26.*: UI test page
• src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/EditorTextChangedIOS26.cs: NUnit test

Issues Fixed

Fixes dotnet#32651

Original prompt

copilot --allow-all-tools --allow-all-paths
/agent pr-reviewer
please review dotnet#32653

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