Fix HttpPart not respecting part name

.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 () => {