Fix: restrict Metal foveated rendering to VisionOS

[ValDesu]

Prevents standard iOS and macOS Metal targets from incorrectly enabling SUPPORTS_FOVEATED_RENDERING_NON_UNIFORM_RASTER.

Fixed error

Shader error in 'Toon/Toon': undeclared identifier 'RemapFoveatedRenderingLinearToNonUniform' at /Library/PackageCache/com.unity.render-pipelines.core@9d79d0cd2cf7/ShaderLibrary/FoveatedRendering.hlsl(25) (on metal)

This error causes the shader to instantly fall back to the pink internal error shader. Wrapping SHADER_API_METAL with defined(UNITY_VISIONOS) ensures the build target is actually compatible with foveated rendering before defining the macro.

[cla-assistant-unity]

All committers have signed the CLA.

[sindharta-tanuwijaya]

Thank you for your contribution!
We'll merge this PR into an internal branch to run our CI, and we may push additional related edits there. Because of this, your commits may not appear in the master branch even after this PR has been merged, but your changes will be included in an upcoming release if our CI passes.
Thanks again!