Don't reject ambiguous forms until we try to dispatch the submit event

[SteveSandersonMS]

Follow-up on the second half of #49298 (comment)

Currently, the SSR renderer will throw during the initial page load if it tries to render multiple named events with clashing names. We can change this to not throw unless you actually try to dispatch the event with the ambiguous name.

It should be fairly straightforwards as we just need to update EndpointHtmlRenderer so that instead of tracking the named events in a dictionary name->location, it tracks a name->hashset<location>, and then we can throw if that hashset has > 1 member at the time we come to dispatch it.