test: make menubar imports safe in headless environments

[lyonsno]

Summary

This PR decouples packaging/omlx_app imports from the heavy omlx runtime so menubar-focused tests can run in headless/CI environments without initializing MLX/Metal during test collection.

Previously, importing omlx_app (or modules under it) could import omlx._version, which triggers omlx/init.py side effects and can abort when MLX initializes in non-GUI/headless contexts.

What Changed

1. packaging/omlx_app/init.py

• Replaced direct from omlx._version import version with a lightweight loader that reads omlx/_version.py via
  importlib.util without importing omlx.

2. packaging/omlx_app/app.py

• Switched version import from from omlx._version import version to from . import version so it uses the
  package-local safe loader.

3. tests/conftest.py

• Made omlx.request imports lazy in fixtures (sample_request, sample_request_factory) and used TYPE_CHECKING for annotations, preventing eager MLX import at pytest collection time.

4. tests/test_omlx_app_package_init.py (new)

• Added guard tests ensuring:
  • importing omlx_app does not import omlx* runtime modules
  • importing omlx_app.app (with GUI deps stubbed) does not import omlx* runtime modules

Why

• Keeps menubar app modules importable in environments where MLX/Metal runtime initialization is unavailable or unsafe during test discovery.
• Prevents collection-time crashes and makes menubar/server-manager tests reliable in CI/headless execution.

Behavioral Impact

• No intended runtime behavior change for the app itself.
• Version string resolution remains sourced from omlx/_version.py; fallback is "0.0.0" only if that file/spec cannot be loaded.

Tests

1. /Users/noahlyons/dev/omlx/.venv/bin/python -m pytest tests/test_omlx_app_package_init.py tests/test_omlx_app.py -q
2. Result: 80 passed

Risk / Notes

1. Low risk: changes are import-path and test-harness focused.
2. The only functional code path touched in app runtime is version import plumbing.

[jundot]

Closing this since #350 already includes all changes from this PR (import safety, conftest lazy imports, test_omlx_app_package_init.py). No need to merge both.