Support raw executables with platform-style file extensions

[albertodebortoli]

Pull Request Title

Support raw executables with platform-style file extensions

Description

• Some tools ship as raw executables with a platform-style extension (e.g. smartling.darwin, tool.linux) rather than as an archive or a bare file with no extension. The pre-download URL extension check in Downloader rejected these with an "unsupported file type" error before any download even occurred.
• Removed the extension allowlist from Downloader. File type validation now happens entirely post-download via magic-byte detection in FileTypeDetector (Mach-O, ELF, PE), which already existed. ToolInstaller.unknownFileType covers truly unrecognisable files.
• No downstream changes needed: FileType.executable, installExecutable(), and desiredBinaryName support were already in place.

Example Lucafile that now works:

tools:
  - name: Smartling
    version: "2.5"
    url: https://github.com/Smartling/smartling-cli/releases/download/2.5/smartling.darwin
    desiredBinaryName: smartling

Type of Change

• Feature

How Has This Been Tested?

• Added / updated unit tests
• Tested on macOS (arch: arm64)

swift build and swift test (374 tests) pass locally.

New test: test_downloadRelease_executable_platformExtension_succeeds — asserts a .darwin-extension URL passes through without error.
Removed test: test_downloadRelease_unsupportedFileType_throws — behaviour no longer lives in Downloader.
Removed test: downloaderError_errorDescription in ErrorDescriptionTests — error type no longer exists.

Checklist

• Swift code builds locally (swift build)
• Tests pass locally (swift test)
• Code style / formatting respected
• Documentation updated (README / comments)

Breaking Changes?

• No

Additional Notes

DownloaderError and SupportedFileTypes have been removed entirely from Downloader. Any truly unsupported file (e.g. a .dmg) will now fail later with ToolInstallerError.unknownFileType instead of the former DownloaderError.unsupportedFileType.