[Python] Fix serialization name for multipart (#9138)

fix #9137

Related PR: 
- [ ] #9139

Author: msyyc

.chronus/changes/python-multipart-fix-serialization-name-2025-11-4-7-44-27.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http-client-python"
+---
+
+Fix serialization name for multipart

packages/http-client-python/emitter/src/types.ts

@@ -235,7 +235,10 @@ function emitProperty(
   }
   return {
     clientName: camelToSnakeCase(property.name),
-    wireName: property.serializationOptions.json?.name ?? property.name,
+    wireName:
+      (property.serializationOptions?.multipart
+        ? property.serializationOptions?.multipart?.name
+        : property.serializationOptions?.json?.name) ?? property.name,
     type: getType(context, sourceType),
     optional: property.optional,
     description: property.summary ? property.summary : property.doc,