[WebGPU] Enable Cast to int64 by default (#28804)
### Description
Support casting to int64 from float32 via IEEE-754 bit decomposition.
- Introduce a new `float_to_int64` helper that emits the
truncated-toward-zero value in full int64 range.
- `to_` type now always allows int64, regardless of `enable_int64`;
casting *from* int64 stays gated by `enable_int64`.
- Adds `cast_op_test.cc` coverage for the newly introduced conversions.
### Motivation and Context
While running the mask-generation vision encoder (`Xenova/sam-vit-base`)
on the WebGPU EP via Transformers.js, float32-to-int64 cast nodes fall
back to the CPU provider under the default session configuration,
because casting to int64 was previously gated behind `enable_int64`
flag, introducing host memcpy and synchronization overhead.
Making cast-to-int64 correct across the full int64 range lets it run on
the
WebGPU EP by default, keeping these nodes on-device and eliminating the
stalls.
### Performance Impact
Measured on the `vision_encoder.onnx` of `Xenova/sam-vit-base`
(mask-generation, SAM ViT-base vision encoder) on the
WebGPU EP.
| Platform | Latency reduction | Speedup |
|------------------|------------------:|--------:|
| Intel Wildcat Lake | −22.8% | 1.30× |
| Intel Panther Lake | −17.1% | 1.21× |
This change yields a 1.2–1.3× speedup on the SAM ViT-base vision encoder
under default configuration.
