The first-pass JSON-decode hardening exposed two latent bugs that the
fuzz scenario hits as soon as the WS handler stays alive past a bad
message:
1) `data.get("type")` is called on whatever `receive_json()` decodes,
but valid JSON can be a list/string/number, not just a dict. Reject
non-object payloads with a structured bad_message error before
dispatch.
2) `submit_answer` did `if answer not in {"A","B","C","D"}` which
raises TypeError when the client sends an unhashable answer
(e.g. a dict). Add an isinstance(str) guard so any non-string
answer falls into the bad_answer branch instead of crashing the
handler.
31/31 pytest still passes. Together with the prior commit, the WS
handlers now survive the full set of fuzz payloads without dropping
the connection.
bb070a688d