Add contains operator (case-insensitive substring)

[ajpallares]

Checklist

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

Motivation

Adds the khepri-specific contains operator for case-insensitive substring checks in workflow predicates (SDK-4348). Companion iOS PR: RevenueCat/purchases-ios#6943

Description

Implemented as function(haystack, needle) with locale-independent lowercasing; non-string haystacks return false, with no json-logic falsy guard on the haystack. Twenty shared predicate fixtures added (fixture count 347 → 367).

[ajpallares]

Closing this PR. We've decided to only implement operators that are part of the JSON Logic specification in the SDKs. contains is a khepri-specific operator and is not part of the JSON Logic spec, so we will not be adding it to the SDK.