[WEB-4491] Make discount code APIs stable

[nicfix]

Why

The public purchases-js fields used to show, prefill, and observe Web Billing discount codes were still marked experimental even though they are now part of the supported purchase and paywall API surface. Keeping those markers made the stability commitment unclear for developers integrating discount-code flows.

What changed

• Remove the experimental marker from discountCode, showDiscountCodeField, and onDiscountCodeChanged on both PurchaseParams and PresentPaywallParams.
• Regenerate the public API report so the published documentation matches the source declarations.

Compatibility

This changes API documentation and stability status only. Field names, types, optionality, and runtime behavior remain unchanged. The internal GetOfferingsParams.discountCode field remains internal and experimental.

Linear: WEB-4491

[RevenueCat-Danger-Bot]

	1 Error
🚫	Label the PR using one of the change type labels. If you are not sure which label to use, choose pr:other.

Label	Description
pr:feat	A new feature. Use along with pr:breaking to force a major release.
pr:fix	A bug fix. Use along with pr:force_minor to force a minor release.
pr:other	Other changes. Catch-all for anything that doesn't fit the above categories. Releases that only contain this label will not be released. Use along with pr:force_patch, or pr:force_minor to force a patch or minor release.
pr:RevenueCatUI	Use along any other tag to mark a PR that only contains RevenueCatUI changes
pr:next_release	Preparing a new release
pr:dependencies	Updating a dependency
pr:phc_dependencies	Updating purchases-hybrid-common dependency
pr:changelog_ignore	The PR will not be included in the changelog. This label doesn't determine the type of bump of the version and must be combined with pr:feat, pr:fix or pr:other.

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