WebGPU: Support int64 for Reshape (#29830)
### Description
This updates WebGPU `Reshape` to follow the existing `enable_int64`
factory-registration pattern (used by `Unsqueeze`/`Expand`/etc.) instead
of static macro registration, and adds explicit int64 WebGPU test
coverage for `Reshape`.
### Motivation and Context
PR
[microsoft/onnxruntime#27478](https://github.com/microsoft/onnxruntime/pull/27478)
added int64 support for the Expand and Unsqueeze (Squeeze) operators in
the WebGPU EP. However, the WebNN EP maps a number of those
shape-manipulation operators — including Squeeze/Unsqueeze — onto
Reshape (WebNN spec doesn't support Squeeze/Unsqueeze). As a result,
when a model uses int64 data through these WebNN-mapped ops, execution
falls back because Reshape itself does not yet accept int64 tensors,
undermining the int64 coverage that #27478 was intended to enable.
Fix #29756
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