Fix keymap help float to respect NormalFloat styling

[Copilot]

The keymap help floating window body was rendered with Normal instead of NormalFloat, causing theme-inconsistent appearance for users with customized float highlight groups. This change aligns the help popup body highlight with standard floating window styling.

• Highlight mapping fix

  • Updated the keymap help float winhighlight to map NormalFloat to NormalFloat (instead of Normal), so the popup body inherits expected float colors.

• Regression coverage

  • Added a focused spec to assert the help window opens with winhighlight = "NormalFloat:NormalFloat".

-- before
vim.wo[win].winhighlight = "NormalFloat:Normal"

-- after
vim.wo[win].winhighlight = "NormalFloat:NormalFloat"

• Fixes [bug] floating keymap help window doesn't respect NormalFloat (1 line fix) #559