Public API XML documentation is not enforced #28

The SDK style guide says XML documentation is required on every public and protected member and that CS1591 is an error, but the root build props suppress CS1591 and many public members have no XML doc comments. This lets public API documentation drift and leaves IntelliSense incomplete for public SDK consumers.

The relevant locations are Directory.Build.props:11, docs/style-guide.md:189, src/Arcp.Client/ArcpClient.cs:34, src/Arcp.Client/ArcpClient.Jobs.cs:13, and src/Arcp.Core/Messages/MetricBody.cs:12.

Fix prompt: Remove CS1591 from NoWarn for shipping projects and add XML documentation for all public and protected members in src. Keep test projects exempt if needed through tests/Directory.Build.props. Use Microsoft Learn style summaries that describe the contract in present tense, add param and returns elements for non-trivial members, and include remarks for lifetime or thread-safety-sensitive types such as transports, clients, sessions, and job handles. Run dotnet build ARCP.slnx and fix all documentation warnings before completing the change.