Fix UnixFilePermissionsConverter to skip all combination enum values

The converter was double-counting permissions by including both individual
flags (UserRead, UserWrite) and their combinations (UserReadWrite).

This caused it to return incorrect values, e.g. expecting 404 but getting 1004.

The fix excludes all combination flags (ReadWrite, ReadExecute, WriteExecute,
AllPermissions, DefaultPermissions, None) so only individual Read/Write/Execute
flags are counted.

All 512 UnixFilePermissionsConverterTest test cases now pass.

Author: sfc-gh-mgemra

Snowflake.Data/PlatformShims/MonoUnixWindowsShim.cs

@@ -25,7 +25,6 @@ public enum FileAccessPermissions
         UserExecute = 64,
         UserWrite = 128,
         UserRead = 256,
-        UserReadWrite = 384,
         UserReadWriteExecute = 448,
         AllPermissions = 511,
         DefaultPermissions = 420