feat(paywalls): add web_view SDK variables provider (PWENG-98)

[alexrepty]

Checklist

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

Motivation

When web content sends rc:request-variables, the SDK replies with safe, already-available system context. v1 exposes only locale; the paywall's dashboard-defined custom variables are intentionally not passed across the bridge.

Part of PWENG-98.

Description

• Adds PaywallWebViewVariablesProvider: builds the SDK-managed variables payload ({ locale } only) and sanitizeAppProvidedVariables, which strips reserved top-level keys so app-provided values cannot overwrite SDK-managed ones (they nest under custom).
• Internal; consumed by the JS bridge in the next PR.
• Tested by PaywallWebViewVariablesProviderTest.

iOS parity: mirrors purchases-ios (web-view-bridge-wiring), which likewise exposes locale only and treats custom variables as out of scope for v1.
Stack (merge bottom-up): schema → rendering → CSP → value types → message model → variables provider → JS bridge → wiring.

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Note

Low Risk
New internal helper and unit tests only; no public API or bridge wiring yet, with deliberate limits on what crosses the web view boundary.

Overview
Adds internal PaywallWebViewVariablesProvider so the upcoming JS bridge can answer rc:request-variables with a v1-safe payload: SDK-managed locale only (no dashboard custom vars, keys, or PII).

sdkManagedVariables(locale) returns { "locale": … } as PaywallWebViewValue map; sanitizeAppProvidedVariables strips reserved top-level keys (today just locale) and logs when apps try to overwrite them—aligned with app vars living under custom via PaywallWebViewController.postVariables.

PaywallWebViewVariablesProviderTest covers locale-only SDK map and reserved-key stripping. Wiring into the bridge is a follow-up PR in the stack.

^{Reviewed by Cursor Bugbot for commit f341b57. Bugbot is set up for automated code reviews on this repo. Configure here.}

[alexrepty]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat(paywalls): wire web_view messaging into the paywall (PWENG-98) #3653
• feat(paywalls): add web_view JavaScript bridge (PWENG-98) #3655
• feat(paywalls): add web_view SDK variables provider (PWENG-98) #3663 👈 (View in Graphite)
• feat(paywalls): add web_view message model and parsing (PWENG-98) #3654
• feat(paywalls): add web_view message value types (PWENG-98) #3662
• feat(paywalls): isolate web_view content with a fixed CSP (PWENG-98) #3652
• feat(paywalls): render web_view component (PWENG-98) #3651
• feat(paywalls): add web_view component schema (PWENG-98) #3650
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[emerge-tools]

📸 Snapshot Test

2 modified, 591 unchanged

Name	Added	Removed	Modified	Renamed	Unchanged	Errored	Approval
TestPurchasesUIAndroidCompatibility com.revenuecat.testpurchasesuiandroidcompatibility	0	0	2	0	333	0	⏳ Needs approval
TestPurchasesUIAndroidCompatibility Paparazzi com.revenuecat.testpurchasesuiandroidcompatibility.paparazzi	0	0	0	0	258	0	N/A

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