Fix for duplicated dockbands (#46438)

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## Summary of the Pull Request

CommandProviderWrapper.PinDockBand had no duplicate check and it blindly
called .Add() on the settings list every time. This allowed the same
extension to be pinned multiple times to the dock. Once duplicates
existed in settings, LoadTopLevelCommands would faithfully re-create all
of them on every CommandsChanged reload, making edit-mode unpin appear
broken (unpin removes one, reload brings them all back).

 **Fix**
- CommandProviderWrapper.PinDockBand: Check all three band lists
(Start/Center/End) for an existing (ProviderId, CommandId) match before
adding; early-return if already pinned.
- CommandProviderWrapper.LoadTopLevelCommands: Track seen command IDs in
a HashSet when iterating AllPinnedCommands; skip duplicates so they
never materialize in the UI even if present in settings.
  
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## PR Checklist

- [ ] Closes: #xxx
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Co-authored-by: Jiří Polášek <[email protected]>