feat(rust): align events with latest protocol

[yukkit]

Summary

• Align the Rust AG-UI core event/types model with the latest protocol events
• Add activity, reasoning, resume, run outcome, metadata, and encryptedValue support
• Update the Rust client event handler/subscriber APIs for the new event shapes
• Refresh Rust SDK docs for the updated event and type surfaces

Testing

• cargo fmt
• cargo test --workspace
• cargo clippy -p ag-ui-core --all-targets -- -D warnings
• cargo clippy -p ag-ui-client --lib --tests

Note: full workspace clippy with all targets still has pre-existing warnings in examples/tests unrelated to this change.

[vercel]

@yukkit is attempting to deploy a commit to the CopilotKit Team on Vercel.

A member of the Team first needs to authorize it.

[MikeSchirtzinger]

verified locally on this branch: cargo test --workspace passes (29 tests), clippy with -D warnings clean on ag-ui-core. the one clippy complaint in ag-ui-client (dead_code in sse.rs) is pre-existing on main and untouched here. this covers the events gap from the plan of record on #972, so I'd love to see it land. LGTM.

the review bottleneck holding this (and the other rust PRs) is now filed as #2256.