Migrate compaction LLM test

[kitlangton]

Summary

• Extend the compaction process test layer helper to accept an LLM layer.
• Convert the first LLM-backed compaction summary test to it.instance and yielded services.
• Keep per-test variation local with Effect.provide(compactionProcessLayer(...)).

Testing

• bun test --timeout 5000 test/session/compaction.test.ts -t "marks summary message as errored|persists tail_start_id|shrinks retained tail|falls back to full summary when even one recent turn|replays the prior user turn|falls back to overflow guidance|publishes compacted event|adds synthetic continue"
• bun typecheck from packages/opencode
• bunx prettier --check packages/opencode/test/session/compaction.test.ts

Stack

• Migrate configurable compaction tests #26732
• Migrate compaction LLM test #26733 👈 current
• Migrate LLM compaction tail tests #26734
• Migrate compaction plugin test #26736
• Migrate compaction tool-call test #26737
• Replace compaction create test fixtures #26738
• Drop compaction create facade #26739
• Drop unused ID Zod statics #26740
• Simplify compaction test helpers #26742