Fix HttpPart not respecting part name (#3909)

fix  [#3779](#3779)

Fixing this issue where the property name always won against the
explicit part name for naming the part over the wire. This was the
opposite of what the design said.

.chronus/changes/fix-multipart-name-2024-6-19-17-57-10.md

@@ -0,0 +1,8 @@
+---
+# Change versionKind to one of: internal, fix, dependencies, feature, deprecation, breaking
+changeKind: fix
+packages:
+  - "@typespec/http"
+---
+
+Fix `HttpPart` not respecting explicit part name by always using the property name

packages/http/src/payload.ts

@@ -311,7 +311,7 @@ function resolveMultiPartBodyFromModel(
   for (const item of type.properties.values()) {
     const part = diagnostics.pipe(resolvePartOrParts(program, item.type, visibility));
     if (part) {
-      parts.push({ ...part, name: item.name, optional: item.optional });
+      parts.push({ ...part, name: part.name ?? item.name, optional: item.optional });
     }
   }
 

packages/http/test/multipart.test.ts

@@ -79,7 +79,7 @@ describe("define with the tuple form", () => {
 });
 
 describe("define with the object form", () => {
-  it("use name from property name", async () => {
+  it("part explicit name is used", async () => {
     const body = await getMultipartBody(`
       op read(
         @header contentType: "multipart/mixed",
@@ -89,7 +89,7 @@ describe("define with the object form", () => {
     `);
 
     strictEqual(body.parts.length, 1);
-    strictEqual(body.parts[0].name, "myPropertyPart");
+    strictEqual(body.parts[0].name, "myPart");
   });
 
   it("using an array of parts marks the part as multi: true", async () => {