hashsum: fix help text to show actual utility name

- Fix all hashsum utilities (md5sum, b2sum, sha256sum, etc.) to show correct utility name in help
- Previously showed 'Usage: hashsum --<digest>' instead of 'Usage: md5sum [OPTION]...'
- Override help_template and usage for specific utilities while preserving hashsum behavior
- Resolves issue where uucore::util_name() returns canonical name 'hashsum' for all variants

Fixes #8614

Author: AnarchistHoneybun

src/uu/hashsum/src/hashsum.rs

@@ -498,7 +498,7 @@ pub fn uu_app_custom() -> Command {
 /// hashsum is handled differently in build.rs
 /// therefore, this is different from other utilities.
 fn uu_app(binary_name: &str) -> (Command, bool) {
-    match binary_name {
+    let (mut command, is_hashsum_bin) = match binary_name {
         // These all support the same options.
         "md5sum" | "sha1sum" | "sha224sum" | "sha256sum" | "sha384sum" | "sha512sum" => {
             (uu_app_common(), false)
@@ -516,7 +516,17 @@ fn uu_app(binary_name: &str) -> (Command, bool) {
         "b3sum" => (uu_app_b3sum(), false),
         // We're probably just being called as `hashsum`, so give them everything.
         _ => (uu_app_custom(), true),
+    };
+    
+    // If not called as generic hashsum, override the command name and usage
+    if !is_hashsum_bin {
+        let usage = format!("{} [OPTION]... [FILE]...", binary_name);
+        command = command
+            .help_template(uucore::localized_help_template(binary_name))
+            .override_usage(format_usage(&usage));
     }
+    
+    (command, is_hashsum_bin)
 }
 
 #[allow(clippy::cognitive_complexity)]