【Hackathon 9th No.58】Unit test for speculate_rebuild_append_padding

[cloudforge1]

Add unit tests for speculate_rebuild_append_padding.

⚠️ Important: XPU-only op — no GPU CUDA kernel

This op only has an XPU implementation. There is no GPU CUDA source in custom_ops/gpu_ops/.

XPU source: custom_ops/xpu_ops/src/ops/mtp/speculate_rebuild_append_padding.cc
XPU test reference: custom_ops/xpu_ops/test/test_speculate_rebuild_append_padding.py

Options:

1. Write a GPU-side test if the op can be invoked through Python bindings on GPU
2. Write an XPU test following the existing XPU test pattern
3. Write a CPU-level reference test if possible

Study the XPU test at custom_ops/xpu_ops/test/test_speculate_rebuild_append_padding.py for the expected behavior.

Test file: tests/operators/test_speculate_rebuild_append_padding.py

Branch: task/058-spec-rebuild-append-test