test(vllm): remove unsupported prefill migration test

[alec-flowers]

Summary

• remove the permanently skipped vLLM prefill-migration test and its stale timing entry
• remove 8 matrix nodes that described a capability Dynamo does not implement
• keep the supported aggregated, KV-transfer, and decode migration tests unchanged

Support evidence and decision

This is intentionally a removal rather than an unskip:

• Dynamo's Migration operator wraps the complete prefill-to-decode pipeline. On a migratable error, RetryManager clones and reissues the PreprocessedRequest through that pipeline.
• The migrator accumulates generated response token IDs. During prefill there are no generated response tokens to preserve, and the vLLM prefill path exposes no checkpoint/export/restore mechanism for partially computed prefill KV state.
• Consequently, a failure during prefill can at most restart and recompute the prompt on another prefill worker. That is request replay/new-request recovery, not migration of in-flight prefill state.
• The test was introduced in test: Rewrite Request Migration Tests and Add Disagg Scenarios #5448 already marked xfail with Prefill migration not yet supported; it was later converted to an unconditional skip. It never represented a supported regression boundary.
• The test's assertion only checked eventual request success, so even an xpass could not distinguish full recomputation from state migration.

A permanently skipped hypothetical test has no regression sensitivity and should not remain in the nightly denominator. If partial prefill-state transfer is implemented later, it needs a new test with an observable state-reuse contract.

Validation

• exact nightly image collection before: 28 tests collected
• exact nightly image collection after: 20 tests collected (8 unsupported prefill nodes removed)
• python3 -m py_compile tests/fault_tolerance/migration/test_vllm.py
• git diff --check
• scoped pre-commit formatting, lint, spelling, and hygiene hooks passed
• repository-wide pytest-marker-report reached the changed file and reported tests/fault_tolerance/migration/test_vllm.py: 20, then failed while collecting unrelated suites because the shared test port allocator was exhausted

No GPU run was needed: this change only deletes an unconditionally skipped test, and production-code inspection conclusively shows that its claimed state-transfer capability is absent.

Stack navigation

1. #13684 — Aggregate migration
2. #13692 — Prefill removal · DYN-4118
3. #13693 — Decode migration · DYN-4119
4. #13694 — KV-transfer migration · DYN-4120

This PR is layer 2 and targets codex/vllm-agg-migration. Review and merge the stack in order.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.