Can `HttpPart` be used in request that content-type is not "multipart/*"?

`HttpPart` is designed content-type `multipart/*` (design issue is [here](https://github.com/microsoft/typespec/issues/3046)), while tsp compiler doesn't report error when it is used in request that content-type is not "multipart/*". (e.g. [Playground](https://typespec.io/playground/?c=aW1wb3J0ICJAdHlwZXNwZWMvaHR0cCI7Cgp1c2luZyBUeXBlU3BlYy5IdHRwOwpAc2VydmljZSgjeyB0aXRsZTogIldpZGdldCBTxhoiIH0pCm5hbWVzcGFjZSBEZW1vxxo7CgogICAgbW9kZWwgTXVsdGlQYXJ0UmVxdWVzdCB7xR0gIHByb2ZpbGVJbWFnZXM6IMR3xCc8Ynl0ZXM%2BO8cmYWRkcmVzc80iQcYUxyR9y23HGslkY2l0eTogc3RyaW5nyC7FBy8vIEBwb3N0yA1vcCB1cGxvYWQoQGhlYWRlciBjb250ZW505AEXOiAibeQAxnBhcnQvZm9ybS1kYXRhIiwgQMkXQm9keSBib2R5OvEA8Sk6IHZvaWTnARXKdkByb3V0ZSgiL25ldyIpxRPpAIZOZXcoLi4u3ksvL%2BcA9kpzb27uAV4vL%2FIBYeUBWOYBBcUf6wFa5wE3yR33ASXsALIy5wCz7AE8TmV3MuQAt%2BsAkOkAsg%3D%3D&e=%40typespec%2Fopenapi3&options=%7B%22linterRuleSet%22%3A%7B%22extends%22%3A%5B%22%40typespec%2Fhttp%2Fall%22%5D%7D%7D))

Then the question is:
- Is it valid that `HttpPart` is used in common content-type?

If no, tsp compiler shall report error.
If yes, how does emitter generate openapi/ SDK? Now the generated openapi.yaml seems not right:


cc @allenjzhang / @timotheeguerin