StreamBuilder: aggregate validation errors

[Jaydbrown]

Instead of throwing on the first invalid field, collect all problems and throw one ValidationError with a .issues[] array.

[ZacLou]

I'd like to work on this issue. I have relevant experience with this tech stack and can deliver a clean, well-tested PR. I can submit within 1-2 days.

[xtep103]

Hi there, I’ve carefully reviewed the issue details and I’m confident I can provide a clean, efficient, and maintainable solution.