[wasi] Path.GetTempFileName and Directory.CreateTempSubdirectory fail: MksTemps/MkdTemp are stubbed on WASI #134943

Description

On WASI, Path.GetTempFileName() always throws, and the exception message says the operation succeeded:

System.IO.IOException : Success : '/tmp/'
   at Interop.ThrowExceptionForIoErrno(ErrorInfo errorInfo, String path, Boolean isDirError)
   at System.IO.Path.GetTempFileName()

GetTempFileName calls SystemNative_MksTemps. wasi-libc provides neither mkstemps nor mkstemp (HAVE_MKSTEMPS and HAVE_MKSTEMP are 0 for wasi), so the shim takes its WASI fallback, which returns -1 without setting errno:

// src/native/libs/System.Native/pal_io.c
#elif TARGET_WASI
    assert_msg(false, "Not supported on WASI", 0);
    result = -1;

SystemNative_MkdTemp, used by Directory.CreateTempSubdirectory, has the same shape: it returns NULL on WASI without setting errno.

The temp directory is usable: with the test harness's --dir=. preopen, /tmp resolves to the app bundle's tmp directory. Only the unique-name creation is missing.

This fails any code that creates temp files. In the CoreCLR WASI library test runs on #134813 it accounts for about 160 failures across System.Data.Common, System.CodeDom, Microsoft.Extensions.Configuration.Functional, System.ComponentModel.Composition, System.ServiceModel.Syndication, System.Diagnostics.TextWriterTraceListener, System.Reflection.Metadata, System.Memory.Data and Microsoft.Extensions.Configuration.FileExtensions.

Proposed fix

Implement both entry points for WASI in pal_io.c using primitives wasi-libc supports, the same way libc implements mkstemp:

• SystemNative_MksTemps: validate the template (at least six X before the suffix, otherwise EINVAL), then loop a bounded number of times:
  • fill the XXXXXX from getentropy (backed by wasi:random), mapped to [A-Za-z0-9]
  • open(pathTemplate, O_RDWR | O_CREAT | O_EXCL, 0600)
  • return the descriptor on success, retry on EEXIST, return -1 with errno preserved on any other error
  • after the attempts run out, fail with EEXIST
• SystemNative_MkdTemp: the same loop with mkdir(pathTemplate, 0700).
• Set errno on every failure path, so a remaining failure produces a meaningful exception instead of "Success".

Filling the template can go in a small helper shared by both entry points. Tests that already cover Path.GetTempFileName and Directory.CreateTempSubdirectory exercise the change once they run on WASI.