Drop SupportedOSPlatform gates; SDK now runs on Linux

The dialect / orchestrators were defensively gated on Windows because
HistorianSspiClient previously P/Invoked InitializeSecurityContextW. With
that replaced by NegotiateAuthentication (cross-platform), the gates are
unnecessary. Removed them from:

- Historian2020ProtocolDialect (4 read paths + 3 status helpers)
- HistorianClient.EnsureTagAsync / DeleteTagAsync
- HistorianWcf{Auth,Read,Event,Status,TagWrite}Orchestrator/Helper
- HistorianWcf{HistAddressing,MessageCapture}Behavior
- HistorianWcfBindingFactory (with #pragma on the Named-Pipe builder
  which still requires Windows at the BCL level)

Runtime constraint: LocalPipe and RemoteTcpIntegrated transports still
require Windows because NetNamedPipeBinding and the Windows transport
security binding are Windows-only at the BCL level. RemoteTcpCertificate
is now usable from Linux, and ProbeAsync is verified working from a
Debian client (10.100.0.35) against the Windows Historian (10.100.0.48).

171/171 tests still pass on Windows.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AVEVA.Historian.Client/HistorianClient.cs

@@ -132,10 +132,6 @@ public sealed class HistorianClient : IAsyncDisposable
     public Task<bool> EnsureTagAsync(HistorianTagDefinition definition, CancellationToken cancellationToken = default)
     {
         ArgumentNullException.ThrowIfNull(definition);
-        if (!OperatingSystem.IsWindows())
-        {
-            throw new ProtocolEvidenceMissingException("EnsureTagAsync requires Windows for the SSPI auth path");
-        }
         return new HistorianWcfTagWriteOrchestrator(_options).EnsureTagAsync(definition, cancellationToken);
     }
 
@@ -150,10 +146,6 @@ public sealed class HistorianClient : IAsyncDisposable
     public Task<bool> DeleteTagAsync(string tagName, CancellationToken cancellationToken = default)
     {
         ArgumentException.ThrowIfNullOrWhiteSpace(tagName);
-        if (!OperatingSystem.IsWindows())
-        {
-            throw new ProtocolEvidenceMissingException("DeleteTagAsync requires Windows for the SSPI auth path");
-        }
         return new HistorianWcfTagWriteOrchestrator(_options).DeleteTagAsync(tagName, cancellationToken);
     }