fix(json-rpc): propagate key parse errors in PubSubItem deserializer

[neowangreal]

The visit_map implementation used while let Ok(Some(key)) to iterate map keys, which causes Err from next_key() to silently break the loop instead of propagating the error. This could lead to misleading downstream errors (e.g., "missing field params") instead of the actual parse error.

inconsistent with all next_value()? calls in the same function, and with the idiomatic serde visitor pattern used elsewhere in the repo.

[neowangreal]

Rebase @mattsse