Add rc.length operator for strings and arrays (#4050)

### Checklist
- [x] If applicable, unit tests
- [ ] If applicable, create follow-up issues for `purchases-ios` and
hybrids

### Motivation
Port of RevenueCat/purchases-ios#7437. String and array length are only
reachable today through awkward `substr` probes or reduce counters,
neither of which supports readable rules or arithmetic on the length.

### Description
- Add `rc.length`: UTF-16 code units for strings (JS `String.length`
parity, including emoji), top-level element count for arrays.
- Anything else, including the null from a missing `var`, throws a type
mismatch rather than returning 0, so an absent value can't match a
length check by accident.
- Fourteen fixtures copied byte-for-byte from iOS (396 total).

### Notes
Stacked on #4047. The fixture JSON is the shared cross-platform spec, so
it is intentionally identical to the iOS file rather than reformatted.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Additive rules-engine operator with strict type errors and shared
fixtures; existing predicates are unchanged unless they start using
`rc.length`.
> 
> **Overview**
> Adds **`rc.length`** so targeting rules can get a numeric length for
strings and arrays (and use it in comparisons/arithmetic).
> 
> Strings use **UTF-16 code units** (JS `String.length` / emoji as 2).
Arrays use top-level element count. Other types, including a missing
`var` resolving to null, throw `TypeMismatch` instead of returning `0`,
so absent data cannot accidentally match a length check.
> 
> Literal arrays must be wrapped (`{"rc.length": [[1,2,3]]}`). Extra
args follow json-logic-js first-arg spread. Covered by 15 shared
predicate fixtures (suite 416 → 431).
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
74a93324deb8496b827c6ceb178ee841809741af. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Cursor <cursoragent@cursor.com>