[http][multipart] missing expected name of HttpPart

[msyyc]

According to #3046:

I expect the "name" of this part shall be "image", however, for the similar scenario :

typespec/packages/http/test/multipart.test.ts

Lines 133 to 154 in 9d48ad1

	it("extract headers for the part", async () => {
	const op = await getHttpOp(`
	op read(
	@header contentType: "multipart/mixed",
	@header operationHeader: string;
	@multipartBody body: [
	HttpPart<{
	@body nested: string,
	@header partHeader: string;
	}>]): void;
	`);
	strictEqual(op.parameters.parameters.length, 2);
	strictEqual(op.parameters.parameters[0].name, "Content-Type");
	strictEqual(op.parameters.parameters[1].name, "operation-header");
	const body = op.parameters.body;
	strictEqual(body?.bodyKind, "multipart");
	strictEqual(body.parts.length, 1);
	strictEqual(body.parts[0].headers.length, 1);
	strictEqual(body.parts[0].headers[0].options.name, "part-header");
	});

current http parse it into "undefined" instead of "nested":

[markcowl]

@bterlson please investigate

[timotheeguerin]

we cut that from the design as it was a bit far fetched.