feat(payment-next): Pass experimentationPreview param through to Nimbus

[david1alvarez]

Because:

• The Checkout Service was hardcoding "false" when fetching Nimbus experiments

This commit:

• Passes through the query param through to the nimbusManager.fetchExperiments calls, like is done in the Glean Service

Closes #PAY-3668

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[Copilot]

Pull request overview

This PR threads the experimentationPreview query parameter through the Payments Next UI → cart fetching flow so the cart/checkout path can pass preview correctly into NimbusManager.fetchExperiments, aligning behavior with the existing Glean metrics service.

Changes:

• Add optional searchParams plumbing to getCart/getSuccessCart actions and validators.
• Propagate searchParams into cart/free-trial eligibility evaluation and map experimentationPreview → Nimbus preview.
• Update one cart service unit test to account for the new optional argument.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
libs/payments/ui/src/lib/nestapp/validators/GetCartActionArgs.ts	Extends validator args to allow optional searchParams.
libs/payments/ui/src/lib/nestapp/nextjs-actions.service.ts	Extends getCart and getSuccessCart to accept/pass optional searchParams.
libs/payments/ui/src/lib/actions/getCartOrRedirect.ts	Passes flattened searchParams into getCart action call (and uses query params for redirects).
libs/payments/ui/src/lib/actions/getCart.ts	Extends server action to accept and forward flattened searchParams.
libs/payments/cart/src/lib/checkout.service.ts	Passes searchParams into getFreeTrialEligibility and maps experimentationPreview to Nimbus preview.
libs/payments/cart/src/lib/cart.service.ts	Extends getCart to accept optional searchParams and forward to free-trial eligibility.
libs/payments/cart/src/lib/cart.service.spec.ts	Updates an expectation to include searchParams: undefined.
apps/payments/next/app/[locale]/error.tsx	Forwards current URL search params into getCartAction.

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[Copilot]

This change introduces new behavior for Nimbus preview selection based on searchParams.experimentationPreview, but there is no unit test asserting that nimbusManager.fetchExperiments is called with preview: true when that param is present. Adding a test case in checkout.service.spec.ts will help prevent regressions in the preview passthrough logic.

[Copilot]

params is derived from the truthiness of searchParams, so callers that pass an empty object (common with Next searchParams) will produce a trailing ? (because urlSearchParams.toString() is empty). Consider deriving params from the serialized query string instead (only prefixing with ? when the string is non-empty).

Suggested change

	const params = searchParams ? `?${urlSearchParams.toString()}` : '';
	const queryString = urlSearchParams.toString();
	const params = queryString ? `?${queryString}` : '';