* [Schema] Omit the id of an error response that never had one
An error response whose id could not be read sent `"id": ""`, which claims
the peer issued a request with an empty-string id - a different statement
from "the id could not be read". JSON-RPC spells the latter as a null id,
and a receiver that cannot read the id is exactly the case the code covers.
[BC Break] Error::$id accepts null and getId() may return it. All the for*()
factories default to null, fromArray() accepts a missing or explicitly-null
id, and the member is omitted from the serialized form when absent.
* Guard null response id in Client/Server Protocol handlers
Also fix Error::jsonSerialize()'s phpdoc: data belongs inside error, not beside it.
* Never let a throwable escape Protocol::processInput()
* Validate payload types in the schema hydration methods
* Stop leaking handler exception messages to the client
* feat(server): add bidirectional client communication support
* fix(server): use correct session ID string conversion method
* fix: clean up docblocks for request handler methods in Builder
* fix: merge artifacts
