Migrate LLM compaction tail tests

[kitlangton]

Summary

• Convert two LLM-backed compaction tail tests to it.instance and yielded services.
• Extend the compaction process test layer helper to cover LLM-backed processors.
• Keep per-test LLM/config variation local with Effect.provide(compactionProcessLayer(...)).

Testing

• bun test --timeout 5000 test/session/compaction.test.ts -t "falls back to full summary when retained tail media|retains a split turn suffix|falls back to full summary when even one recent turn"
• bun typecheck from packages/opencode
• bunx prettier --check packages/opencode/test/session/compaction.test.ts
• bunx oxlint packages/opencode/test/session/compaction.test.ts (existing warnings only, no errors)

Stack

1. Migrate configurable compaction tests #26732
2. Migrate compaction LLM test #26733
3. Migrate LLM compaction tail tests #26734 👈 current
4. Migrate compaction plugin test #26736
5. Migrate compaction tool-call test #26737
6. Replace compaction create test fixtures #26738
7. Drop compaction create facade #26739
8. Drop unused ID Zod statics #26740
9. Simplify compaction test helpers #26742