fix: skip redundant computations in the schema subset checker

[yanjizhong]

Summary

Fixes two redundant recomputations in the JSON Schema subset checker: cached falsy results never hit the cache, and calculateObject computes each optional key expression twice.

Changes

• checker/cache.ts: getWithCache now checks result === undefined instead of !result, so cached falsy results (false and ExpressionNone) are reused. checker/equals.ts already used this form.
• checker/object.ts: the optional-key loop pushes the already computed expression instead of calling calculateKey a second time, matching the adjacent additionalProperties block.

Motivation

• With !result, every deep-equality miss (false) and every ExpressionNone schema comparison bypassed the cache and was recomputed on each lookup.
• The optional-key loop called the recursive calculateKey twice per key purely to obtain the same value.

Testing

• bun run check and bun run test in packages/open-flow. Both changes are behavior-preserving; the existing schema-compare suites cover the affected paths.

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📥 Commits

Reviewing files that changed from the base of the PR and between 6f1e960 and d8a91c3.

📒 Files selected for processing (2)

• packages/open-flow/src/json-schema-subset/checker/cache.ts
• packages/open-flow/src/json-schema-subset/checker/object.ts

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