Fix #22574: fix volumetric fog back-face intersection artifacts

[joaoconceicao12]

Objective

Fixes #22574.

This PR fixes angle-dependent volumetric fog artifacts in the atmosphere example ([--features free_camera], where rotating the camera caused shadow flipping and hard edges.

Solution

The root cause was inconsistent back-face intersection setup between CPU and shader paths:

• CPU side: far-plane construction for fog volume traversal.
• WGSL side: ray-plane intersection math.

This PR:

• Corrects far-plane setup in the volumetric fog CPU path.
• Corrects the WGSL ray-plane intersection calculation.
• Adds a regression unit test for [get_far_planes]) to prevent this from regressing.

Testing

• cargo test -p bevy_pbr --lib passes (including the new regression test).
• Manual visual test:
  • cargo run -r --example atmosphere --features free_camera
  • Rotating the camera no longer shows the previous flipping/hard-edge artifacts. (see video)

video1.mov

Platform tested:
macOS (Metal backend).
No additional untested platform-specific behavior is currently known.

[github-actions]

Your PR caused a change in the graphical output of an example or rendering test. This might be intentional, but it could also mean that something broke!
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