test(server): vary fuzz HMAC keys

fuzz/README.md

@@ -23,7 +23,7 @@ of the root workspace, requires nightly Rust, and is not built by
 | `decode_echo_reply` | `decode_echo_reply` against a deterministic, bounded `Params` matrix, unauthenticated and HMAC-authenticated |
 | `decode_params`     | `Params::decode` and `Params::decode_with_presence`, plus an encode/decode round trip |
 | `decode_varint`     | `varint::decode_uvarint`/`decode_varint` and their round trip through `encode_uvarint`/`encode_varint` |
-| `server_core_datagrams` | A bounded mix of raw datagrams and production-encoded Open/Echo/Close requests against one `ServerCore`; it varies IPv4/IPv6 peers and session/HMAC configuration, authenticates keyed requests, reuses issued tokens, and asserts the session cap |
+| `server_core_datagrams` | A bounded mix of raw datagrams and production-encoded Open/Echo/Close requests against one `ServerCore`; it varies IPv4/IPv6 peers, session configuration, and bounded (up to 128-byte) HMAC keys, authenticates keyed requests, reuses issued tokens, and asserts the session cap |
 
 The invariant every target asserts is: **arbitrary bytes must never panic**.
 Returning `Err` is normal and expected; a successful decode is also normal.

fuzz/fuzz_targets/server_core_datagrams.rs

@@ -17,7 +17,7 @@ const MAX_INPUT_LEN: usize = 32 * 1024;
 const MAX_DATAGRAMS: usize = 16;
 const MAX_DATAGRAM_LEN: usize = 2 * 1024;
 const MAX_SESSIONS: u8 = 8;
-const FUZZ_HMAC_KEY: &[u8] = b"fuzz-server-core-key";
+const MAX_HMAC_KEY_LEN: usize = 128;
 
 #[derive(Clone)]
 struct FuzzSession {
@@ -35,11 +35,16 @@ fuzz_target!(|data: &[u8]| {
     let Ok(mut config) = config(&mut input) else {
         return;
     };
-    let hmac_key = input
-        .arbitrary::<bool>()
-        .ok()
-        .filter(|enabled| *enabled)
-        .map(|_| FUZZ_HMAC_KEY);
+    let hmac_key = match input.arbitrary::<bool>() {
+        Ok(true) => {
+            let Ok(length) = input.int_in_range(1..=MAX_HMAC_KEY_LEN) else {
+                return;
+            };
+            input.bytes(length).ok()
+        }
+        Ok(false) => None,
+        Err(_) => return,
+    };
     if let Some(key) = hmac_key {
         config = config.with_hmac_key(key);
     }