fix: dispatch UnmarshalJSON by implemented interface, not entrypoint

[momomuchu]

Description

Fixes #583.

Decode picked which UnmarshalJSON to call based on whether ContextOption
was set on the call (json.Unmarshal vs json.UnmarshalContext), instead of
checking which interface the concrete type actually implements. If a type only
implements UnmarshalJSON(context.Context, []byte) and gets decoded via plain
json.Unmarshal, the type assertion v.(json.Unmarshaler) panics (and vice
versa for the reverse case).

DecodeStream already handles this correctly: it type-switches on
unmarshalerContext vs json.Unmarshaler and only falls back to
context.Background() when ContextOption isn't set (since c05e1e2, #327).
This brings Decode in line with that, so both paths behave the same way.

Added a test covering both directions plus a check that the plain Unmarshaler
path still returns identical bytes to encoding/json.