[http] Slightly stronger types for multipart resolution.

[witemple-msft]

This PR makes the types returned from @typespec/http for multipart payloads slightly more accurate by splitting the tuple and model forms of multipart bodies into separate types with their own multipartKind and stricter subtypes about what kinds of parts they can have inside them, whether or not they can be named, whether or not they are optional, etc.

The types of the parts are also split into two partKind types: tuple and model. The guarantees given are as follows:

For model-based multipart payloads:

• type is guaranteed to be a Model.
• parts are guaranteed to be only model-based parts.

For tuple-based multipart payloads:

• type is guaranteed to be a Tuple.
• parts are guaranteed to be only tuple-based parts.

For model-based parts:

• name is required.
• property is required and refers to the ModelProperty that defined the part (and we remember to set it -- we weren't doing this correctly).

For tuple-based parts:

• name is optional.
• property is guaranteed to be undefined.

[github-actions]

All changed packages have been documented.

• ✅ @typespec/http-client-js
• ✅ @typespec/http

Show changes

@typespec/http - feature ✏️

Improved types for HTTP multipart payloads for more precise guarantees and additional information about the resolution of individual parts.

@typespec/http-client-js - internal ✏️

Changed MultipartTransform slightly to avoid a bizarre TypeScript type checking error.

[azure-sdk]

You can try these changes here

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