Fix race condition in SharedMemoryHelpers.CreateOrOpenFile for named mutexes on Unix

[Copilot]

Two processes racing to create the same named mutex could hit a TOCTOU window where O_RDWR sees ENOENT but O_CREAT|O_EXCL then fails with EEXIST (another process won the race), producing an unhandled IOException.

Fix: Wrap CreateOrOpenFile in a retry loop — when O_CREAT|O_EXCL fails with EEXIST, loop back and open the now-existing file instead of throwing.

// Before: threw IOException on EEXIST
fd = Interop.Sys.Open(path, O_RDWR | O_CLOEXEC | O_CREAT | O_EXCL, perms);
if (fd.IsInvalid)
{
    error = Interop.Sys.GetLastErrorInfo();
    throw Interop.GetExceptionForIoErrno(error, path); // throws on EEXIST
}

// After: retry when another process beat us to creation
if (fd.IsInvalid)
{
    error = Interop.Sys.GetLastErrorInfo();
    if (error.Error == Interop.Error.EEXIST)
        continue; // loop back to open the existing file
    throw Interop.GetExceptionForIoErrno(error, path);
}

This mirrors the existing retry pattern already used in EnsureDirectoryExists for the same class of race.

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