fix: align rules-engine-internal with json-logic-js (undefined & missing_some)

[ajpallares]

Checklist

• If applicable, unit tests
• If applicable, create follow-up issues for purchases-ios and hybrids

Motivation

json-logic-js is the cross-SDK source of truth for rule evaluation. A conformance suite running the shared predicate fixtures against json-logic-js surfaced three edge cases where rules-engine-internal diverged. iOS counterpart: RevenueCat/purchases-ios#7066.

Description

• Model JS undefined as a distinct Value subclass: {"and": []} / {"or": []} now reduce to undefined (falsy, but undefined !== null) instead of null.
• {"log": []} logs "undefined" instead of "null".
• missing_some treats a string options argument as an array-like list (matching missing.apply(this, options)) instead of throwing.
• Shared predicate fixtures updated to the json-logic-js results.

---

Note

Medium Risk
Changes core rule evaluation semantics (undefined vs null, strict equality, missing_some); wrong behavior could flip rule outcomes, but risk is mitigated by a large expanded conformance fixture suite aligned to json-logic-js.

Overview
Brings rules-engine-internal in line with json-logic-js for edge cases surfaced by the shared predicate conformance suite.

JS undefined as Value.Undefined: Omitted operator args and empty and / or now yield undefined (falsy, but undefined !== null) instead of null. Coercion, truthiness, == / ===, and stringification ("undefined") follow JS; var treats an undefined path like an empty path (full scope) and coerces an undefined default to null.

Operator fixes: missing_some accepts string (and nullish) options like missing.apply—one key for a string, threshold from length (UTF-16)—instead of erroring on non-array options. Iteration operators use undefined for a missing predicate; log / substr match omitted-arg behavior.

Tests: Predicate fixtures updated (382 cases); expectations for empty and/or, equality, var, missing, map/reduce, etc.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.28%. Comparing base (2214c5c) to head (8f462f4).

Additional details and impacted files

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##             main    #3634   +/-   ##
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  Coverage   80.28%   80.28%           
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  Files         379      379           
  Lines       15570    15570           
  Branches     2173     2173           
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  Hits        12501    12501           
  Misses       2204     2204           
  Partials      865      865           

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[tonidero]

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