[Review] [CDX-379] Validate testCells

[govambam]

Recreated from Constructor-io#429 for Macroscope review.

Original PR: Constructor-io#429 by @esezen
Status: open (open)

Recreated using direct PR head fetch - exact current state preserved.

Original PR: Constructor-io#429

[macroscopeapp]

Validate testCells by enforcing non-empty string values in ConstructorIO.constructor and ConstructorIO.setClientOptions using utils.helpers.toValidTestCells in constructorio.js

Introduce utils.helpers.toValidTestCells and apply it to testCells during construction and client option updates; add tests across constructor, modules, and utils to verify filtering and request parameter inclusion.

🖇️ Linked Issues

Resolves CDX-379 by validating testCells values during initialization, updates, and request construction.

📍Where to Start

Start with toValidTestCells in helpers.js, then review its usage in ConstructorIO.constructor and ConstructorIO.setClientOptions in constructorio.js.

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Macroscope summarized 9833c5d.