fix(broadcast): fix issues mentioned in #164

[bmckerry]

Fixes the issues mentioned in comments of #164.

WIP as I'm getting this error when attempting to call copy.deepcopy() on PyAnyMessage:

---- broadcaster::tests::test_message_rejected stdout ----

thread 'broadcaster::tests::test_message_rejected' panicked at src/messages.rs:333:26:
called `Result::unwrap()` on an `Err` value: PyErr { type: <class 'TypeError'>, value: TypeError("cannot pickle 'builtins.PyAnyMessage' object"), traceback: Some("Traceback (most recent call last):\n  File \"/Users/benmckerry/.local/share/uv/python/cpython-3.11.13-macos-aarch64-none/lib/python3.11/copy.py\", line 161, in deepcopy\n    rv = reductor(4)\n         ^^^^^^^^^^^\n") }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    broadcaster::tests::test_message_rejected

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 41 filtered out; finished in 0.01s