Migrate compaction overflow tests

[kitlangton]

Summary

• Convert two overflow compaction process tests to it.instance and yielded services.
• Use SessionCompaction.use.process directly instead of per-test ManagedRuntime wrappers.
• Remove an unused import from the compaction test file.

Testing

• bun test --timeout 5000 test/session/compaction.test.ts -t "replays the prior user turn|falls back to overflow guidance"
• 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

• Migrate compaction overflow tests #26731 👈 current
• Migrate configurable compaction tests #26732
• Migrate compaction LLM test #26733
• 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