Add MLX op handler for aten.bitwise_xor

[nanookclaw]

Summary

• Add BitwiseXorNode to the MLX delegate schema, C++ runtime, Python op handler, and tests
• Enables element-wise bitwise XOR for boolean and integer tensors via mlx::core::bitwise_xor

Closes #18927

Test plan

• python -m executorch.backends.mlx.test.run_all_tests -k bitwise_xor passes both bool and int variants
• Existing MLX op tests remain passing

🤖 Generated with Claude Code

[pytorch-bot]

🔗 Helpful Links

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[metascroy]

@nanookclaw Looks like a great start!

A couple notes:

• Do not check in auto-generated files (e.g., MLXLoader.cpp, MLXLoader.h, etc)
• Can you confirm both the scalar and tensor versions are being tested in your tests?