[Test] Fix Q8KV8 sparse-prefill pool fixture after page-size rename

[Jialin]

Motivation

After #38954 renamed the pool field to swa_page_size, the Q8KV8 sparse-prefill tests still expose swa_window_size on their mock pool. The backend raises AttributeError before the workspace comparison can run.

Modifications

Update the mock field and its get_extra_key_page_size getter to use swa_page_size. Two-line, test-only fix; no production changes.

Accuracy Tests

Ran the existing test file on CUDA before and after the fix:

pytest -q test/registered/kernels/ops/attention/test_q8kv8_sparse_prefill_backend.py --tb=short --disable-warnings

• Before, on main 40a84d6dfc: 3 failed, 9 passed, 2 skipped.
• After: 12 passed, 2 skipped. All three helper cases (compression ratios 0, 4, 128) pass.
• The two real-kernel tests require SM90 and were skipped on the available hardware; SM90 validation remains for CI.
• pre-commit run --files test/registered/kernels/ops/attention/test_q8kv8_sparse_prefill_backend.py: passed with the local base set to upstream/main.

Speed Tests and Profiling

Not applicable: no runtime or kernel changes.

Checklist

• Formatting and applicable pre-commit checks pass.
• Existing regression tests verified before and after the fix.
• Test-only change; no documentation updates required.
• Follows SGLang code style.

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CI States

Latest PR Test (Base): ❌ Run #34565335083
Latest PR Test (Extra): ❌ Run #34565334771
Latest PR Test (AMD ROCm 10): ❌ Run #34565335140