feat(app): allow hexadecimal HMAC key specification and redact keys from debug output

crates/irtt-app/src/shared/client/args.rs

@@ -26,8 +26,9 @@ pub struct CommonClientArgs {
     #[arg(long, default_value_t = 0, value_parser = parse_length)]
     pub length: u32,
 
-    /// HMAC key.
-    #[arg(long, value_parser = parse_hmac_key)]
+    // Decode during configuration preparation so Clap cannot echo rejected keys.
+    /// HMAC key: literal UTF-8 or lowercase 0x followed by hexadecimal bytes.
+    #[arg(long)]
     pub hmac: Option<String>,
 
     /// Clock mode to request.
@@ -267,13 +268,6 @@ pub fn parse_length(input: &str) -> Result<u32, String> {
     Ok(length)
 }
 
-pub fn parse_hmac_key(input: &str) -> Result<String, String> {
-    if input.is_empty() {
-        return Err("HMAC key must not be empty".to_owned());
-    }
-    Ok(input.to_owned())
-}
-
 pub fn parse_server_fill(input: &str) -> Result<String, String> {
     if input.is_empty() {
         return Err("server fill must not be empty".to_owned());