docs(api-server): add /ws WebSocket endpoint + SubscribeMessage to OpenAPI (#1164)

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Summary

The router registers five routes (/health, /simulate, /routes, /routes/:name, /ws), but only the first four appear in the utoipa-derived OpenAPI spec surfaced at /openapi.json and the Swagger UI at /docs. websocket::ws_handler had no #[utoipa::path(...)] annotation, and SubscribeMessage (the JSON shape clients send to subscribe to a tx_id on /ws) did not derive ToSchema. Anyone browsing the generated docs has no indication that /ws exists, let alone how to use it.

Fix

• ws_handler gains a #[utoipa::path(get, path = "/ws", ...)] annotation documenting the upgrade (utoipa represents protocol upgrades as a plain GET for discovery).
• SubscribeMessage (types.rs) now derives ToSchema and gets per-field doc comments explaining the action ("subscribe" / "unsubscribe") and tx_id fields.
• ApiDoc (main.rs) lists crate::websocket::ws_handler in paths(...) and crate::types::SubscribeMessage in components(schemas(...)).

No behavior change. The handler and protocol are unchanged; the change is purely about surfacing them in the generated spec.

I do not have a local Rust toolchain on this machine, so I could not run cargo test myself, but the diff is annotation-only and ToSchema derive on a simple Clone + Serialize + Deserialize struct — please run the test suite on a Soroban environment before merging.

Closes

#1164