iOS: Make GPU validation mode configurable in Xcode schemes

[coderabbitai]

Current State

The GPU validation mode is currently enabled by default in the shared Xcode scheme for all builds.

      debugDocumentVersioning = "YES"
      debugServiceExtension = "internal"
+     enableGPUValidationMode = "1"

Problem

Having GPU validation mode enabled by default in shared schemes can:

• Introduce measurable CPU overhead during development
• Impact performance in production builds
• Affect all developers using the shared scheme, even when not debugging GPU-related issues

Proposed Solution

Consider one of these approaches:

1. Move GPU validation mode to a debug-specific scheme
2. Make it configurable via build configuration
3. Document when to enable/disable it in the development workflow

Implementation Notes

• Review current GPU validation settings across all schemes
• Create separate debug schemes with GPU validation enabled
• Update documentation to reflect GPU validation usage guidelines

References

• Original PR: Ios config #2490
• Discussion: Ios config #2490 (comment)

Additional Context

This change aligns with best practices for Metal API validation in iOS development, where validation tools should be used diagnostically rather than enabled by default in production builds.