Fix encode/decode todo's (#13683)

Mistakes have been made. I forgot about a bunch of `todo`s in the helper
functions. So, this PR replaces them with proper errors. It also adds
tests for parse-time evaluation, because one `todo` I missed was in a
`run_const` function.

crates/nu-command/tests/commands/base/mod.rs

@@ -2,13 +2,15 @@ use data_encoding::HEXUPPER;
 use rand::prelude::*;
 use rand_chacha::ChaCha8Rng;
 
+use nu_test_support::nu;
+
 mod base32;
 mod base32hex;
 mod base64;
 mod hex;
 
 /// Generate a few random binaries.
-pub fn random_bytes() -> Vec<String> {
+fn random_bytes() -> Vec<String> {
     const NUM: usize = 32;
     let mut rng = ChaCha8Rng::seed_from_u64(4);
 
@@ -17,8 +19,26 @@ pub fn random_bytes() -> Vec<String> {
             let length = rng.gen_range(0..512);
             let mut bytes = vec![0u8; length];
             rng.fill_bytes(&mut bytes);
-            let hex_bytes = HEXUPPER.encode(&bytes);
-            format!("0x[{}]", hex_bytes)
+            HEXUPPER.encode(&bytes)
         })
         .collect()
 }
+
+pub fn test_canonical(cmd: &str) {
+    for value in random_bytes() {
+        let outcome = nu!("0x[{}] | encode {1} | decode {1} | to nuon", value, cmd);
+        let nuon_value = format!("0x[{value}]");
+        assert_eq!(outcome.out, nuon_value);
+    }
+}
+
+pub fn test_const(cmd: &str) {
+    for value in random_bytes() {
+        let outcome = nu!(
+            r#"const out = (0x[{}] | encode {1} | decode {1} | encode hex); $out"#,
+            value,
+            cmd
+        );
+        assert_eq!(outcome.out, value);
+    }
+}