A .NET client for AHP

Go, Rust, Swift, Kotlin, and TypeScript all made it to the party. C#, the objectively superior language, somehow didn't get the invite. So here's a .NET client to fix that, and I'd like to be the one who maintains it.

It's at parity with the other five: same shared round-trip and reducer corpus, green CI, generated off the same TS types with the freshness check in CI, no runtime dependencies, System.Text.Json throughout.

The real question with a 6th client is who maintains it, so to be clear, I'm not handing it off for someone on your side to pick up. I'll keep it at parity as the protocol changes, fix bugs, and handle the issues. The codegen makes that genuinely light: a protocol change is one regen, with nothing to track.

Publishing: the package IDs are Microsoft.AgentHostProtocol.* (yours). I'll wire the pack-and-publish workflow to match your Kotlin/TS lane, leaving the signing key on your side.

It's up as a draft in #206 (356 tests, Release-clean, current with main). If you want a .NET client, I'm glad to own it.

@connor4312 @benibenj

Review the draft implementation in #206 and compare it with the existing Go, Rust, Swift, Kotlin, and TypeScript clients. Check the shared round-trip and reducer corpus, generated TypeScript types, freshness check, and the 356-test result; done means parity, green CI, and a matching pack-and-publish workflow.