EditNetSpell: Avoid unhandled-key sound for cursor keys

[mstv]

Fixes #10715

Proposed changes

Mark cursor key presses as handled if the EditNetSpell's RichTextBox will not handle them
in order to suppress the unhandled-key sound ("ding")
primarily when editing the commit message
This is done in dependency on the current cursor position because the KeyEventArgs.Handled flag is not set (yet) if the RichTextBox handles the key.

Test methodology

• manual

Test environment(s)

• Git Extensions 33.33.33
• Build 6400348
• Git 2.39.0.windows.1
• Microsoft Windows NT 10.0.19045.0
• .NET 6.0.12
• DPI 96dpi (no scaling)
• Microsoft.WindowsDesktop.App Versions

    Microsoft.WindowsDesktop.App 6.0.12 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Merge strategy

I agree that the maintainer squash merge this PR (if the commit message is clear).

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✒️ I contribute this code under The Developer Certificate of Origin.

[RussKie]

What's the lifetime of EditNetSpell?

[mstv]

What's the lifetime of EditNetSpell?

The same as the injected RichTextBox.
The event handlers reference it.

[mstv]

What's the lifetime of EditNetSpell?

The same as of the injected RichTextBox. The event handlers reference it.

Does this cause problems, @RussKie?

[RussKie]

No, I was curious whether we need to unbind handlers and dispose. All good, thanks

[mstv]

I want to merge this soon in order to resolve the regression caused by the first attempt #10715. All PR builds are affected: The cursor keys do not work in the commit message editor.
Can I get approvals?