fix(client-dotnet): apply lenient TLS to GalaxyRepositoryClient and enforce hostname on CA-pin

Mirror MxGatewayClient's three-branch handler structure in GalaxyRepositoryClient
(CA-pin / lenient accept-all / OS trust) so the Galaxy endpoint works against the
gateway's self-signed cert under the default lenient posture. Expose an internal
CreateHttpHandlerForTests seam for unit testing. Add RemoteCertificateNameMismatch
rejection at the top of both CA-pinned callbacks so a pinned-CA connection truly
verifies the host. Strengthen existing lenient test to invoke the callback and assert
it returns true; add mirrored Galaxy-client handler tests.

clients/dotnet/ZB.MOM.WW.MxGateway.Client/MxGatewayClient.cs

@@ -338,6 +338,11 @@ public sealed class MxGatewayClient : IAsyncDisposable
                 X509Certificate2 trustedRoot = X509CertificateLoader.LoadCertificateFromFile(options.CaCertificatePath);
                 handler.SslOptions.RemoteCertificateValidationCallback = (_, certificate, chain, errors) =>
                 {
+                    if ((errors & System.Net.Security.SslPolicyErrors.RemoteCertificateNameMismatch) != 0)
+                    {
+                        return false;
+                    }
+
                     if (certificate is null)
                     {
                         return false;