Fix device_each to skip instead of fail when no matching device

[Nir-Az]

Summary

• PR Fail tests instead of skipping when required device is missing #14854 changed pytest.skip to pytest.fail when a required device is missing, which is correct for device() marker
• But it also affected device_each() which should skip when no matching device exists (e.g. D585S on a Jetson with only D401)
• Fix: only pytest.fail when a device() marker is present; device_each()-only tests skip gracefully

Test plan

• pytest-ah-configurations.py (device_each D585S) skips when no D585S connected
• pytest-backend-vs-frame-timestamp.py (device D400*) fails when no D400 connected

[Copilot]

Pull request overview

Adjusts device-resolution behavior in the unit-test pytest plugin so device_each()-only tests skip (not fail) when no matching hardware is connected, while preserving the stricter “missing required device == fail” behavior for device().

Changes:

• Detect whether a device() marker is present when no devices match.
• pytest.fail only when device() is present; otherwise pytest.skip for device_each()-only cases.
• Keeps the existing “all matching devices excluded” case as a skip.

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