Bump package version to 0.1.0-preview.6 (#206)

Author: eiriktsarpalis

src/Directory.Build.props

@@ -6,7 +6,7 @@
     <RepositoryUrl>https://github.com/modelcontextprotocol/csharp-sdk</RepositoryUrl>
     <RepositoryType>git</RepositoryType>
     <VersionPrefix>0.1.0</VersionPrefix>
-    <VersionSuffix>preview.5</VersionSuffix>
+    <VersionSuffix>preview.6</VersionSuffix>
     <Authors>ModelContextProtocolOfficial</Authors>
     <Copyright>© Anthropic and Contributors.</Copyright>
     <PackageTags>ModelContextProtocol;mcp;ai;llm</PackageTags>