Find SwiftPM metallib bundle near test binary

[RNT56]

Summary

• search the test binary directory and its parents for the SwiftPM resource bundle
• handle the case where Bundle.allBundles already contains the SwiftPM resource bundle itself
• keep the existing main-bundle, bundle-resource, framework, colocated, and METAL_PATH fallback behavior

Validation

• git diff --check upstream/main...upstream-pr/swiftpm-metallib-bundle-lookup
• downstream mlx-swift SwiftPM resource branch builds MLX and produces platform-specific default.metallib resources; runtime loading of those resources depends on this lookup change

[RNT56]

This is the small MLX-side lookup change needed for the SwiftPM default.metallib resource work in mlx-swift.

It keeps the existing lookup behavior and adds the SwiftPM resource bundle case. That should let the mlx-swift packaging PR use an upstream MLX commit once this lands.

Local validation:

• git diff --check upstream/main...upstream-pr/swiftpm-metallib-bundle-lookup
• downstream mlx-swift packaging branch builds MLX and produces platform-specific default.metallib resources

[RNT56]

I realized this is effectively the same automatic lookup approach as #3562, which was already superseded by the explicit metallib path API in #3597. I’ll close this and move the SwiftPM packaging work to that API route instead.