Always return Content-MD5 header and JSON field on successful writes

Go always sets Content-MD5 in the response regardless of whether the
request included it. The Rust version was conditionally including it
only when the request provided Content-MD5.

seaweed-volume/src/server/handlers.rs

@@ -2068,7 +2068,6 @@ pub async fn post_handler(
 
     // Validate Content-MD5 if provided
     let content_md5 = content_md5.or(parsed_content_md5);
-    let has_request_content_md5 = content_md5.is_some();
     if let Some(ref expected_md5) = content_md5 {
         if expected_md5 != &original_content_md5 {
             return json_error_with_query(
@@ -2288,13 +2287,13 @@ pub async fn post_handler(
             } else {
                 // H2: Use Content-MD5 computed from original uncompressed data
                 let content_md5_value = original_content_md5;
-                // Match Go: only include contentMd5 in response when request provided Content-MD5
+                // Match Go: always include contentMd5 in response JSON and header
                 let result = UploadResult {
                     name: filename.clone(),
                     size: original_data_size, // H3: use original size, not compressed
                     etag: n.etag(),
                     mime: mime_type.clone(),
-                    content_md5: if has_request_content_md5 { Some(content_md5_value.clone()) } else { None },
+                    content_md5: Some(content_md5_value.clone()),
                 };
                 let etag = n.etag();
                 let etag_header = if etag.starts_with('"') {
@@ -2305,10 +2304,8 @@ pub async fn post_handler(
                 let mut resp = (StatusCode::CREATED, axum::Json(result)).into_response();
                 resp.headers_mut()
                     .insert(header::ETAG, etag_header.parse().unwrap());
-                if has_request_content_md5 {
-                    resp.headers_mut()
-                        .insert("Content-MD5", content_md5_value.parse().unwrap());
-                }
+                resp.headers_mut()
+                    .insert("Content-MD5", content_md5_value.parse().unwrap());
                 resp
             }
         }