Publishing an incomplete event does not return an error

[matt2e]

• Publishing to a topic has no checks for whether the event can be unmarshalled (eg missing value for field)
• When subscriptions try consuming the event is when the errors start appearing
• This is more of a publishing error not a subscription error so we should try to surface this at the time of publication

Steps to repro

Write the following code:

//ftl:data
type Event struct {
    Id string `json:"id"`
    Name string `json:"name,omitempty"`
}

var Topic = ftl.Topic[Event]("topic")
var _ = ftl.Subscription(Topic, "sub")

//ftl:verb
func Publish(ctx context.Context) error {
    err := Topic.Publish(ctx, Event{
        Id: "abc",
    })
    // err should be set because the event is not valid, but it is nil
    return err
}

//ftl:subscribe sub
//ftl:retry 3 5s 30s
func ProcessEvent(ctx context.Context, event Event) error {
    // never reached
    return nil
}

When Publish() is called, FTL creates the event but fails each time it tries to consume it.
So for one published event, we get all 4 attempts logged like this:

warn:sub:example.sub:example.processEvent: Async call could not be called: could not validate HTTP request body: example.Event.name is required
warn:sub:example.sub:example.processEvent: Async call could not be called: could not validate HTTP request body: example.Event.name is required
warn:sub:example.sub:example.processEvent: Async call could not be called: could not validate HTTP request body: example.Event.name is required
warn:sub:example.sub:example.processEvent: Async call could not be called: could not validate HTTP request body: example.Event.name is required