test: restore prefill fixture compatibility

[PhilipJohnBasile]

What changed

Add the missing _supports_skip_lm_head capability stub to the lightweight scheduler fixture in test_step_prefill_reclaims_before_first_guard.

Root cause

e3c6912b taught _step_prefill_chunk to query _supports_skip_lm_head() before calling the model. The production Scheduler defines that helper, but this test invokes the unbound method on a minimal SimpleNamespace and did not add the new capability to its fixture.

As a result, all three CI jobs for jundot/omlx#2568 fail at the same test with AttributeError, after 7,982 other tests pass.

Impact

Test fixture only. Production behavior, DeepSeek-V4 kernels, cache semantics, and performance code are unchanged. Returning False keeps this ordering test on the stock model-call branch it exercised before the optimization.

Verification

• Negative control: PR #2568 CI fails this exact test on Python 3.11, 3.12, and 3.13 because the fixture lacks the helper.
• Patched focused regression: 1 passed.
• Full affected file: 45 passed.
• git diff --check: passed.
• Independent read-only Sol review of the frozen one-line diff: approved with no blockers.

Prepared with Codex and independently reviewed before publication.