Fix maxNumberOfServerInstances==1 implying first-instance semantics on Unix for NamedPipeServerStream

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On Windows, maxNumberOfServerInstances: 1 automatically sets FILE_FLAG_FIRST_PIPE_INSTANCE, so a second process attempting to bind the same pipe name gets UnauthorizedAccessException. On Unix, first-instance semantics were only applied when PipeOptions.FirstPipeInstance was explicitly set -- a second process with maxNumberOfServerInstances: 1 would silently Unlink() the existing socket and rebind, hijacking the pipe.

Root cause: In SharedServer.Get, isFirstPipeInstance was derived solely from PipeOptions.FirstPipeInstance, ignoring maxCount.

Fix: One-line change in NamedPipeServerStream.Unix.cs -- treat maxCount == 1 as first-instance, matching Windows behavior:

// Before
bool isFirstPipeInstance = (pipeOptions & PipeOptions.FirstPipeInstance) != 0;

// After
bool isFirstPipeInstance = (pipeOptions & PipeOptions.FirstPipeInstance) != 0 || maxCount == 1;

With this change, when a cross-process second server creation is attempted with maxNumberOfServerInstances: 1, socket.Bind() fails against the existing socket (no pre-unlink), and the existing SocketException catch converts it to UnauthorizedAccessException.

Test added: NamedPipe_MaxOneInstance_Throws_WhenNameIsUsedAcrossProcesses in NamedPipeTest.CrossProcess.cs -- uses RemoteExecutor to verify cross-process enforcement.

Note: Orphaned sockets after a server crash remain a known limitation on Unix (vs. Windows kernel objects which auto-clean). This equally affects PipeOptions.FirstPipeInstance and is pre-existing behavior.

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