Fix copyOnSelect right-click overwriting clipboard with stale selection (#19943)

## Summary of the Pull Request
Fix `copyOnSelect` right-click paste overwriting the clipboard with a
stale selection instead of pasting the current clipboard contents.

## Detailed Description of the Pull Request / Additional comments
When `copyOnSelect` is enabled, the selected text is already copied to
the clipboard on left mouse button release in `PointerReleased`. The
selection is intentionally left visible as a visual reference (the
`_selectionNeedsToBeCopied` flag is set to `false` to track that the
copy already happened).

However, the right-click handler in `PointerPressed` unconditionally
called `CopySelectionToClipboard()` before pasting, ignoring both the
`copyOnSelect` setting and the `_selectionNeedsToBeCopied` flag. This
caused any still-visible selection to be re-copied to the clipboard,
overwriting whatever the user may have copied from another application
(or another terminal tab) in the meantime.

This change splits the right-click behavior based on the `copyOnSelect`
setting:
- **`copyOnSelect: true`** — Skip the redundant copy, clear the
selection, and paste directly. The text was already copied on mouse-up.
- **`copyOnSelect: false`** — Preserve existing behavior: copy the
selection (if any), clear it, and paste only if there was no selection
to copy.

## Validation Steps Performed
1. Set `"copyOnSelect": true` in `settings.json`.
2. Selected text in a terminal pane - verified it was copied to
clipboard on mouse-up.
3. Switched to Notepad, copied different text ("NEW TEXT").
4. Switched back to Terminal (selection still visible), right-clicked —
verified "NEW TEXT" was pasted, not the old selection.
5. Verified right-click with no active selection still pastes clipboard
contents.
6. Verified right-click immediately after selecting (no app switch)
pastes the just-selected text.
7. Set `"copyOnSelect": false` — verified right-click still copies
selection first, then pastes only when no selection exists (original
behavior unchanged).
8. Verified tab-switching with an active selection does not cause stale
clipboard overwrites on right-click.

## PR Checklist
Closes #14465 (dupe of #19942)

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Co-authored-by: Sagar Bhure <sagarbhure@microsoft.com>