`nominal_hash_rate` is accepting NaN and others

[lucasbalieiro]

messages that have nominal_hash_rate (F32) is accepting weird values that are not suitable for a hashrate information. messages affected include: UpdateChannel and OpenChannel;

a test example below:

diff --git a/sv2/subprotocols/mining/src/update_channel.rs b/sv2/subprotocols/mining/src/update_channel.rs
index f2f728cb..25d44348 100644
--- a/sv2/subprotocols/mining/src/update_channel.rs
+++ b/sv2/subprotocols/mining/src/update_channel.rs
@@ -89,3 +89,28 @@ impl fmt::Display for UpdateChannelErrorOwned {
         )
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use alloc::vec;
+    use binary_sv2::Deserialize;
+
+    // Spec 5.3.7: nominal_hash_rate represents hashrate in h/s and must be >= 0.
+    // The parser currently accepts NaN for f32 fields, which is a spec violation.
+    #[test]
+    fn test_update_channel_accepts_nan_hashrate() {
+        let mut data = vec![
+            0x00, 0x00, 0x00, 0x00, // channel_id: 0
+        ];
+        // nominal_hash_rate: NaN (0x7FC00000 in LE)
+        data.extend_from_slice(&f32::NAN.to_le_bytes());
+        data.extend_from_slice(&[0u8; 32]); // maximum_target
+        let parsed = UpdateChannel::from_bytes(&mut data)
+            .expect("parser should accept NaN hash rate (spec 5.3.7 violation)");
+        assert!(
+            parsed.nominal_hash_rate.is_nan(),
+            "nominal_hash_rate should be NaN to demonstrate the bug"
+        );
+    }