Freezing a message does not freeze bytes fields, allows modifying the message after the freeze

We represent `bytes` fields as `Uint8List`, which doesn't support freezing. Repro:

```proto
syntax = "proto3";

message M1 {
bytes b = 1;
}
```

```dart
void main() {
M1 m = M1();
m.b = [0];
m.freeze();
m.b.add(1);
print(m);
}
```

Start from the Dart message freeze entry point and the handling of bytes fields represented by Uint8List. Reproduce the issue with the provided M1 example, then verify that mutating m.b after m.freeze() is prevented and add coverage for that behavior.

Written by the indexing model from the issue text.