Guard kqueue observer on unsupported platforms

[shuofengzhang]

What changed

• Guard kqueue observer selection when the platform lacks kqueue support.
• Skip the explicit kqueue test on platforms without kqueue support.

Why

• Avoids AttributeError when watchdog imports kqueue on non-kqueue platforms (e.g., Linux).
• Keeps explicit kqueue coverage on supported platforms while making the test suite portable.

Testing

• ✅ pytest -q (pass)