docs: Add XML documentation comments to public API

[josedab]

Description

Public API classes need XML doc comments for IntelliSense support.

Task

• Add <summary>, <param>, <returns>, and <example> XML doc comments
• Focus on StreamlineClient, StreamlineProducer, StreamlineConsumer
• Enable <GenerateDocumentationFile>true</GenerateDocumentationFile> in .csproj

Helpful Context

• C# XML docs guide