feat: match array properties element-wise for non-set operators

[kyeh-amp]

Summary

Adds element-wise (any-match) evaluation for multi-valued user properties when paired with non-set operators (is, contains, greater, etc.), aligning the local evaluation engine with Amplitude analytics/charts behavior.

Previously, non-set operators always stringified the whole property value via coerce_string — so an array property like ["a","b"] would never satisfy is "a". Now:

• match_condition calls coerce_string_array for every non-null value. If the value is multi-valued (native array or JSON array string) and the operator is non-set, it delegates to a new match_strings_non_set helper that returns true if any element satisfies the operator.
• Negation operators (is not, does not contain) also use any-match semantics — is not "A" on ["A","B"] is true because "B" is not "A".
• Scalars still fall through to the existing coerce_string → match_string path unchanged.

Also fixes two issues in coerce_string_array:

• Added a start_with?('[') pre-check so scalar strings skip JSON.parse and avoid exception-driven control flow on every evaluation.
• Corrected a pre-existing bug where scalars (non-array JSON or malformed JSON) were returned as [value] instead of nil, which caused set operators to spuriously match single-string properties.

Tests

• New spec/experiment/evaluation/evaluation_spec.rb with an inline test harness (flag_with_condition, context_with_prop, evaluate, assert_match, assert_no_match) and 14 unit cases covering scalars, native arrays, JSON array strings, malformed JSON, and leading-whitespace edge cases.
• 7 new integration cases added inline in the existing operator tests block; the deployment key is consolidated to server-VVhLULXCxxY0xqmszXouXxiEzoeJWmSh (a superset of the prior key, containing all existing flags plus the new multi-value flags).

Full suite: 271 examples, 0 failures. Rubocop: clean.

Checklist

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• Does your PR have a breaking change?: No

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Note

Medium Risk
Changes the local flag evaluation logic for non-set operators when targeting multi-valued properties, which can alter rollout/targeting results for existing flags. Also refactors evaluation classes into the AmplitudeExperiment::Evaluation namespace, so any missed references could cause runtime load/name errors.

Overview
Local flag evaluation now supports element-wise (any-match) targeting for multi-valued user properties when using non-set operators (e.g. is, contains, comparisons), including handling JSON array strings; this also tightens coerce_string_array to avoid parsing non-array scalars and to stop treating malformed/non-array JSON as a singleton array.

The evaluation subsystem is namespaced and consolidated under AmplitudeExperiment::Evaluation via a new lib/experiment/evaluation.rb entrypoint, with updates to LocalEvaluationClient and specs, plus expanded test coverage (new unit spec and added integration cases) for the new array-matching behavior.

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[kyeh-amp]

Superseded by #92, which is cut cleanly from latest main without unrelated stale commits.