Add Rust managed settings client wrapper

[jbrown1618]

Summary

• Add Client::read_managed_settings() as a top-level Rust SDK helper for the server-scoped managedSettings.read JSON-RPC method.
• Add an opaque ManagedSettings JSON result type so the SDK preserves the runtime-owned canonical payload without reinterpreting policy.
• Cover the helper with an in-memory JSON-RPC test that verifies it sends managedSettings.read with no session ID before any session.create call.

Dependency

This draft depends on the companion github/copilot-agent-runtime draft PR that exposes the server-scoped managedSettings.read method. The generated RPC schema in this repo does not yet include that method, so this PR adds the Rust top-level wrapper directly rather than hand-editing generated RPC output.

github/copilot-host will consume this Rust SDK wrapper to include managed settings during environment registration.

Validation

• rustfmt direct check using nightly-2026-04-14 rustfmt on changed Rust files
• cargo test --test managed_settings_test with Rust 1.94.0
• cargo clippy --all-features --all-targets -- -D warnings with Rust 1.94.0

Limit: cargo test --all-features was attempted but stopped after several minutes because it appeared stuck downloading the bundled runtime package (@github/copilot-darwin-arm64).