Update on "Arm backend: Add 16A8W support and test for add operation"

Add 16A8W quantization support and comprehensive tests for the add operation in ExecutorTorch ARM backend targeting Ethos U55 and U85 NPUs.

This follows the pattern established for linear operations, extending int16 support to add operations with hardware-specific testing.

Changes:
- Add INT16 dtype validation support in op_add.py
- Add test_add_tensor_16a8w_tosa_INT test function with U55/U85 pipeline support
- Add U55 and U85 specific 16A8W tests with proper xfail decorators
- Fix U55/U85 test parameter usage (remove unsupported tosa_extensions, clean quantizer function calls)
- Update xfail reasons to consistent 'Vela compilation fails with Invalid arguments' pattern

Differential Revision: [D80510463](https://our.internmc.facebook.com/intern/diff/D80510463)

cc digantdesai freddan80 per zingo oscarandersson8218

[ghstack-poisoned]

Author: Ninja91