fix: handle SSE events without explicit message event

[paoloricciuti]

Background

SSE events should default to message as the event...both the SSE transport and the HTTP transport were explicitly checking for event message which means that servers that didn't explicitly emit events with an event name where hanging forever.

Summary

I've expanded the check to invoke the onmessage callback even when the event is undefined which is how SSE events should work.

Manual Verification

I've patched my version of ai sdk with the same changes and used a server that wasn't returning explicit event messages and verified that it worked.

Checklist

• Tests have been added / updated (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• A patch changeset for relevant packages has been added (for bug fixes / features - run pnpm changeset in the project root)
• I have reviewed this pull request (self-review)

Related Issues

Closes #10915