[CoreML EP] Add GatherND builder

[maxwbuckley]

Summary

New ML Program op builder: ONNX GatherND → CoreML gather_nd.

• batch_dims must be 0 — the iOS15 gather_nd op has no batch_dims parameter;
  IsOpSupportedImpl rejects other values.
• CoreML's gather_nd rejects a bool x, but transformer attention-mask
  graphs gather from bool tensors. For bool data the builder lowers the op as
  cast(bool→int32) → gather_nd → cast(int32→bool); int32 represents 0/1 exactly,
  so the round-trip is lossless.
• validate_indices is passed explicitly — the ML Program parser rejects
  gather_nd without it (the same quirk the gather builder works around).
• ML-Program-only; IsOpSupportedImpl rejects the NeuralNetwork format.

Depends on the bool-Cast PR

The bool-data GatherND test needs Cast as the int ↔ bool producer/consumer so
the bool tensors stay internal to the CoreML partition (a partition cannot have
bool I/O). This branch is stacked on coreml-cast-bool — the cb43b7c75f
commit in this PR is the bool-Cast PR and drops from this diff once that one
merges.

Tests (coreml_basic_test.cc)

• GatherND_MLProgram — a float GatherND runs on CoreML, matches CPU.
• GatherNDBoolData_MLProgram — a Cast → GatherND → Cast bool chain runs fully
  on CoreML, exercising the cast round-trip lowering.
• GatherNDNeuralNetworkNotSupported — GatherND falls back on the NeuralNetwork
  format.
• GatherNDBatchDimsNotSupported — GatherND with batch_dims=1 falls back to CPU.

Doc: coreml_supported_mlprogram_ops.md lists GatherND.

Series — CoreML EP coverage for transformer / diffusion graphs

• [CoreML EP] Support bool Cast in ML Program #28595 — Support bool Cast in ML Program (prerequisite)
• [CoreML EP] Add Sin and Cos unary ops #28596 — Add Sin and Cos unary ops (independent)
• [CoreML EP] Add Where and And builders #28597 — Add Where and And builders (depends on [CoreML EP] Support bool Cast in ML Program #28595)
• [CoreML EP] Add GatherND builder #28598 — Add GatherND builder (this PR — depends on [CoreML EP] Support bool Cast in ML Program #28595)

Together with #28278 (scalar-Gather), the series takes BERT / GPT-2 / ViT /
diffusion-UNet graphs — tiny and full-size — from 2 CoreML partitions to 1, with
zero graph breaks.