fix: compare single-branch oneOf schemas correctly

[yanjizhong]

Summary

oneOf with a single effective branch (for example oneOf: [{ type: 'string' }], or a two-branch list where one branch compiles to never) compared as compatible against unrelated schemas.

Changes

• expression/calculator.ts: calculateOneOf returns the single expression directly when only one branch remains, because oneOf([X]) is equivalent to X.
• expression/calculator.ts: calculateCombination now throws on an empty expression list. The previous length < 0 guard could never trigger.
• test/designer/schema-compare.test.ts: regression cases for single-branch oneOf in both compatible and incompatible directions.

Motivation

With one expression, the ban-zone computation evaluated AnyOf([]), which returned ExpressionNone, and the defensive ExpressionContainedBy fallback in calculateNode then turned the empty result into a contained verdict. For example, compareJSONSchema({ oneOf: [{ type: 'string' }] }, { type: 'number' }) reported compatible, so a string output could be connected to a number input in the designer.

The compiler rejects empty oneOf lists and folds all-never lists into the never kind, so a combination reaching calculateOneOf always keeps at least one branch after never rejection; the empty-list guard is therefore unreachable hardening.

Testing

• bun run check and bun run test in packages/open-flow.
• New table cases: single-branch oneOf to a matching type (compatible) and to a mismatched type (incompatible).

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📥 Commits

Reviewing files that changed from the base of the PR and between 6f1e960 and b0dc59c.

📒 Files selected for processing (2)

• packages/open-flow/src/json-schema-subset/expression/calculator.ts
• packages/open-flow/test/designer/schema-compare.test.ts

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