Add return value documentation to public APIs

[daviburg]

Problem

The copilot-instructions.md currently says "Do NOT document return values" (no <returns> tag). This convention is inherited from the internal BPM repo and makes sense there, but for a public SDK consumed by external developers, return value documentation is important for discoverability and IDE experience.

Proposed Changes

1. Update .github/copilot-instructions.md to remove the "Do NOT document return values" rule
2. Add <returns> tags to all public methods in src/
3. Run dotnet format to verify no new analyzer warnings

Context

The internal BPM convention avoids <returns> to reduce noise. On public NuGet packages, however, these docs surface in IntelliSense and are part of the developer experience. All three public repos (SDK, LSP, Samples) inherited this rule and should be corrected.