hugr-py does not encode FuncDefn visibility in model export

[aborgna-q]

#2436 added better roundtrip tests that write the test hugr in a given format, convert them to a different one using hugr convert if necessary, and load them back.

These work fine for json -> json, but if we enable writing hugrs in model-exts format then many tests fail with errors like

FAILED hugr-py/tests/test_hugr_build.py::test_simple_id - RuntimeError:   2025-07-17T10:11:34.474358Z ERROR hugr: Error decoding HUGR envelope.

Caused by:
    0: failed to import hugr
    1: import failed in context: `define-func` node with id 0
    2: No visibility for FuncDefn
    at hugr-cli/src/main.rs:38

That means we successfully produce an encoded model envelope, but it fails to load from the rust side in the hugr convert call.

To reproduce this, add model-exts to this list

hugr/hugr-py/tests/conftest.py

Lines 189 to 191 in 1a301eb

	# TODO: "model-exts" comes with its own variety of errors.
	# Fix them and add it to the list of write formats.
	WRITE_FORMATS = ["json"]

And run with just test-python -k test_simple_id.

[aborgna-q]

Looks like this error may only be happening locally for me.
The tests are encoding the model envelopes using an older version of the rust bindings -.-