Implement SEP-2577: Deprecate Roots, Sampling, and Logging

[localden]

Tracking implementation of SEP-2577 for the 2026-07-28 MCP spec release.

[piyushbag]

@guglielmo-san You are assigned on this one, so checking before I open a PR.

I would like to take SEP-2577 for go-sdk if you are not already working on it. My plan is advisory-only deprecation (no wire or behavior change), following the rust-sdk implementation in modelcontextprotocol/rust-sdk#884 and the csharp-sdk Obsolete surface in modelcontextprotocol/csharp-sdk#1651:

• Mark the user-facing Roots, Sampling, and Logging APIs with Go Deprecated comments (session methods, client/server options handlers, capability helpers).
• Document deprecated capability types and fields in protocol.go; keep wire types functional.
• Update generated README/docs sources where those sections are documented.
• Add //lint:ignore or test-file allowances only where the SDK exercises deprecated APIs internally (conformance server, sampling tests), matching rust's #![allow(deprecated)] pattern.

Local gate I would run before opening the PR: go test ./..., go test -race ./..., and both conformance scripts.

Happy to wait for your go-ahead, or to pair if you already have a branch in progress. Fixes #995 when ready.

[guglielmo-san]

@piyushbag thanks, you can take this task