test(lotus_json): assert LotusJson<Option<T>> is nullable in schema

[0xDevNinja]

Summary

Closes #4331.

LotusJson<Option<T>> delegates its schema to Option<T::LotusJson>, which schemars already renders as nullable ("type": [..., "null"] / a null branch in anyOf). This adds a small regression test locking that behaviour in, as suggested in the issue — verifying that:

• LotusJson<Option<T>> produces a schema that permits null (i.e. is optional in the OpenRPC spec), and
• the non-optional LotusJson<T> counterpart does not permit null.

The helper schema_allows_null accepts both the "type" union form (used for primitives/arrays) and the anyOf/oneOf form (used for $ref object types), so the test stays robust across inner types.

No production code changes — test only.

Reference tests

cargo test -p forest-filecoin --lib lotus_json::opt:: — 3 passed (existing snapshot + quickcheck, plus the new optional_is_nullable_in_schema).

Other information and links

Issue originally raised by @ruseinov in #4330 (comment).

Summary by CodeRabbit

• Tests
  • Added regression coverage confirming optional Lotus JSON values accept null.
  • Confirmed non-optional Lotus JSON values continue to reject null.

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 8482053a-d875-4765-b7dc-d27736e9d53b

📥 Commits

Reviewing files that changed from the base of the PR and between 17c49db and de41e74.

📒 Files selected for processing (1)

• src/lotus_json/opt.rs

🔗 Linked repositories identified

CodeRabbit considers these linked repositories for cross-repo context during reviews:

• filecoin-project/lotus (manual)

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Walkthrough

Changes

LotusJson schema nullability

Layer / File(s)	Summary
Schema nullability regression coverage src/lotus_json/opt.rs	Adds schema_allows_null and verifies that LotusJson<Option<Cid>> accepts null, while LotusJson<Cid> does not.

Estimated code review effort: 2 (Simple) | ~10 minutes

Suggested reviewers: hanabi1224, sudo-shashank

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: a regression test for nullable schema handling in LotusJson<Option>.
Linked Issues check	✅ Passed	The added test verifies that LotusJson<Option> is represented as nullable/optional in the schema, matching issue #4331.
Out of Scope Changes check	✅ Passed	The PR only adds a focused regression test and helper logic in the test file, with no unrelated feature or production changes.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

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• Create PR with unit tests

✨ Simplify code

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