[Backport release/0.0.14] fix(camera): select V4L2 for Linux device paths

[github-actions]

Description

Backport of #4159 to release/0.0.14.

[greptile-apps]

Confidence Score: 5/5

Safe to merge: the changed webcam configuration and backend-selection behavior work across the supported identifier and platform cases.

What we checked:

• An independent validation script was run against the PR source using CPython 3.12.14 and reported 10 passing mocked WebcamConfig-to-cv2 flows with exit code 0.
• The focused webcam backend test module completed successfully with 12 passing tests in the same validation run.
• The validation confirmed the path selection behavior: the previous integer-only configuration rejected /dev/video0, while the updated implementation accepts the path and selects the expected backend.
• Evidence artifacts were prepared to enable review of the validation outcomes and the baseline comparison.

Summary

This update expands webcam configuration to accept device paths, normalizes numeric string identifiers into integer indices, and selects the V4L2 backend for Linux /dev/ camera paths while retaining automatic backend selection for numeric indices, streams, and other platforms.

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