[Tests][WebGPU] Update grid-pack test TIRx syntax

[Nanmur]

Description

The grid-pack guard regression test added in #20424 uses TIRx syntax that had already been replaced on main:

• [REFACTOR][Script] Give S-TIR its own construction namespace #20437 removed the s_tir=True arguments from TIRx decorators.
• [REFACTOR][TIRx] Use tensor names for types and allocation APIs #20519 renamed the TIRx parameter annotation from T.Buffer to T.Tensor.

As a result, the current CPU CI fails during test collection with:

AttributeError: No script namespace 'Buffer'

This test-only change updates the new test to the current TIRx syntax. The WebGPU test body and generated-code assertion are unchanged.

The same upstream failure is currently blocking unrelated PRs #20529 and #20535.

Testing

• python -m py_compile tests/python/codegen/test_target_codegen_webgpu.py
• python -m pre_commit run --files tests/python/codegen/test_target_codegen_webgpu.py

A matching current-main native library was not available locally, so the targeted WebGPU runtime test is left to CI.