[AdvancedPaste] Harden ToJsonFromXmlOrCsvAsync against clipboard read failures (#48124)

## Summary

`ToJsonFromXmlOrCsvAsync` in `AdvancedPaste/Helpers/JsonHelper.cs`
documents that it never throws and returns an empty string on any
failure. The clipboard read at the top of the method
(`clipboardData.GetTextAsync()`) was not wrapped, so a transient
clipboard failure could surface as an exception to callers, contrary to
the documented contract.

This PR:

- Wraps `GetTextAsync()` in a try/catch and returns `string.Empty` on
failure, matching the pattern already used by the JSON/XML/CSV parsing
branches further down in the same method.
- Updates the matching unit test to decode input bytes as UTF-8
(`Encoding.UTF8.GetString(input)`) and consume the awaited task via
`GetAwaiter().GetResult()`, for consistency with sibling tests elsewhere
in the solution.

## Validation

- Local build of `AdvancedPaste.sln`. (Note: my machine has a
pre-existing NuGet SDK resolver issue unrelated to this change — the
same baseline fails on `main` for me. CI should be the source of truth.)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>