Fix Piper Linux espeak symlink

[Ironship]

Summary

• Add the missing Linux Piper symlink for libespeak-ng.so.1 after unpacking Piper.
• Keep the existing libpiper_phonemize.so.1 symlink behavior and use version-independent *.so.1.* source matching so future Piper archive bumps do not require exact filename updates.
• Preserve the previous ln -sf style behavior via ln -sfn, so existing or dangling symlinks are replaced instead of blocking setup.
• Add focused regression tests for the Piper Linux symlink patterns, versioned source discovery, and generated ln -sfn command.

Addresses discussion #10659.

Verification

• Checked for existing PRs/fixes matching #10659, libespeak-ng.so.1, and Piper symlink wording; did not find an active duplicate.
• dotnet test .\tests\UI\UITests.csproj -c Debug --no-restore --filter DownloadTtsViewModelTests -v minimal passed: 3/3.
• dotnet build .\src\ui\UI.csproj -c Debug --no-restore succeeded.
• dotnet test .\tests\UI\UITests.csproj -c Debug --no-restore -v minimal passed: 127/127.

Notes

• I verified this on Windows with unit/build tests; the actual symlink creation path is Linux-only and runs after Piper is unpacked.
• AI assistance was used to identify, implement, and verify this change.

[Copilot]

Pull request overview

This PR updates the Piper download flow on Linux so the unpacked engine gets the expected shared-library symlinks via .NET file APIs instead of shelling out to ln. It fits into the UI text-to-speech download pipeline by adjusting DownloadTtsViewModel, which prepares the locally installed Piper runtime before the engine is used.

Changes:

• Added a Linux symlink mapping for libespeak-ng.so.1 alongside the existing libpiper_phonemize.so.1 mapping.
• Replaced the shell-based symlink creation logic with File.CreateSymbolicLink plus explicit cleanup of existing paths.
• Added a focused unit test that verifies the Piper Linux symlink mapping contains both expected library entries.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/ui/Features/Video/TextToSpeech/DownloadTts/DownloadTtsViewModel.cs	Refactors Linux Piper symlink setup and adds the missing libespeak-ng mapping.
tests/UI/Features/Video/TextToSpeech/DownloadTts/DownloadTtsViewModelTests.cs	Adds a regression test that checks the exported Linux symlink mapping.