Migrate compaction plugin test

[kitlangton]

Summary

• Convert the plugin-controlled auto-continue compaction test to it.instance and yielded services.
• Use the per-test compaction layer helper for plugin overrides.

Testing

• bun test --timeout 5000 test/session/compaction.test.ts -t "allows plugins to disable synthetic continue prompt"
• 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
4. Migrate compaction plugin test #26736 👈 current
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