fix(engine): keep an explicit null consolidate output instead of restoring the raw payload

[BaaRai]

Fixes #104

The settle path picked the consolidated output with ??:

output = consolidated.output ?? raw;

Nullish coalescing treats an explicit null as absent, but the usage.consolidate contract only gives absence the meaning of "payload unchanged". null is valid Json (zJson includes it) and a consolidator can return it deliberately when everything it touched was billing. In that case the engine restored the raw vendor payload and re-exposed exactly the fields the hook meant to strip.

The fix falls back to the raw response only when consolidated.output is undefined:

output = consolidated.output === undefined ? raw : consolidated.output;

Added a regression test next to the other D27 settle tests: a consolidate fn returning { credits: {}, output: null } against a successful response now settles with output: null instead of the raw object. Verified the test fails on the old line and passes with the fix; deno task check, the full deno task test suite (1246 passed) and deno task version:check (no contract-surface change, engine bump not required) are green.

Summary by CodeRabbit

• Bug Fixes
  • Consolidated results that explicitly return null now preserve that value instead of falling back to the raw response.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between c57aa3d and 975efbc.

📒 Files selected for processing (2)

• engine/engine.test.ts
• engine/engine.ts

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📝 Walkthrough

Walkthrough

LoadedEndpoint.settle now uses the raw response only when usage.consolidate returns output: undefined. An explicit output: null is preserved. A regression test covers this case.

Priority: ⬇️ Low

Severity of issue fixed: Low

Merge Risk: ⚪ Minimal · up to 975ef

Explicit null output is preserved, while omitted output still falls back to the raw response. No concrete merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the primary change: preserving an explicit null consolidated output instead of falling back to the raw payload.
Linked Issues check	✅ Passed	Issue #104 requires explicit null output preservation, undefined-only fallback, and a regression test. The PR summary reports the settle change from nullish coalescing to an undefined check and adds t…
Out of Scope Changes check	✅ Passed	The reported changes are limited to the settle-path implementation and its regression test. Both changes directly support issue #104. No unrelated change is identified.

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