Handle SMB URIs in Linux desktop_drop path parsing

[Copilot]

Linux drops can include smb://... entries, but the current parser unconditionally calls Uri.toFilePath(), which throws for SMB URIs and causes dropped items to be discarded. This change updates Linux drop parsing to preserve SMB URIs while keeping existing file-URI behavior unchanged.

• Linux drop URI handling

  • In performOperation_linux (packages/desktop_drop/lib/src/channel.dart), parse each dropped line once and branch by scheme:
    • smb → keep original URI string
    • others (e.g. file) → continue converting via toFilePath()
  • This prevents exceptions for network share drops while preserving existing local-path conversion.

• Focused regression coverage

  • Added packages/desktop_drop/test/channel_linux_test.dart to cover:
    • SMB URI passthrough (smb://server/share/file.txt)
    • File URI conversion (file:///tmp/file.txt → /tmp/file.txt)

final uri = Uri.tryParse(e);
if (uri == null) return '';
if (uri.scheme == 'smb') return e;
return uri.toFilePath();