fix: repair the test suite on main

[abendrothj]

Problem

uv run pytest — the command in CLAUDE.md's pre-commit checklist — does not currently run on main. It aborts during collection:

ERROR src/exo/download/tests/test_cancel_download.py
... 8 errors during collection ...
190 deselected, 8 errors in 2.60s

Two independent causes:

1. src/exo/download/ has no __init__.py. Every other test package in the tree has one. Without it, pytest walks up only as far as src/exo/download/ when deriving a module name, so the eight files under src/exo/download/tests/ are imported as tests.test_*. With pythonpath = ".", the repo's top-level tests/ package wins that name and the imports fail:

ModuleNotFoundError: No module named 'tests.test_safetensors_index'

The failure only appears in a full-suite run — pytest src/exo/download/tests alone passes, which is likely why it went unnoticed.

2. rust/exo_rs/tests/test_python.py calls NetworkingHandle.new() with three arguments. The zenoh migration (#2132) added a namespace parameter:

pub fn new(
    identity: &str,
    namespace: &str,
    listen_port: u16,
    discovery_service_port: u16,
) -> PyResult<PyNetworkingHandle> {

so the test raises TypeError: NetworkingHandle.new() missing 1 required positional argument: 'discovery_service_port'.

Fix

Add the missing __init__.py, and pass a namespace in the test.

Result

	before	after
uv run pytest	8 collection errors, aborts	471 passed, 3 skipped

The 68 tests under src/exo/download/tests/ had not been running as part of a full-suite invocation.