Feature Request: Linter Rule for Implicit HttpParts Usage

[joheredi]

Clear and concise description of the problem

Since we now have a way to explicitly define HttpParts (#3046), I think it would be beneficial to introduce a linter rule that warns users when relying on implicit parts. This would encourage clearer and more maintainable specs by making HttpParts usage explicit.

Additionally, a multipart operation must have a @bodyProperty property. Operations that use spread models or are structured like this:

op doThing(
  @header contentType: "multipart/form-data",
  name: HttpPart<string>,
  age: HttpPart<int32>,
  description?: HttpPart<string>,
): void;

are not resolved as multipart in @typespec/http, as discussed in #4041.

Given this behavior, I believe it's important to add a linter rule to help users successfully define multipart operations. This would prevent unexpected behavior and ensure specs are correctly structured.

cc: @timotheeguerin

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