Simplify compaction test helpers

[kitlangton]

Summary

• Collapse duplicate compaction-part helpers introduced during test migration.
• Use the summary compaction helper consistently in migrated tests.

Testing

• bun test --timeout 5000 test/session/compaction.test.ts test/session/schema-decoding.test.ts -t "summarizes only the head|anchors repeated compactions|keeps recent pre-compaction|ignores previous summaries|accepts minimal session"
• 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
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 👈 current