feat: host the server-delivered mobile core (JS brain) in JavaScriptCore with native fallback

[pandeymangg]

Ref ENG-2900

What & why

Was: The survey display decision (display types, recontact days, segments, display percentage, language resolution) is compiled into the SDK; changing it requires an SDK release plus a customer app store release.

Now: setup() fetches a small JS bundle from the Formbricks server (/js/mobile/v1/core.umd.cjs), hosts it in JavaScriptCore, and track() asks it which survey to show. Any failure — no bundle, wrong protocol version, evaluation error — falls back to the untouched native logic, so behaviour without a bundle is identical to today.

First iOS PR of the JS-brain epic (targets epic/js-brain, not main). Server side: formbricks/formbricks#9152.

Where to look

• Sources/FormbricksSDK/MobileCore/MobileCoreRuntime.swift — JSContext host, serial-queue confined, protocol-version gate
• Sources/FormbricksSDK/MobileCore/MobileCoreLoader.swift — fetch + per-host cache, offline fallback
• Sources/FormbricksSDK/Manager/SurveyManager.swift — brain-first track(), native path unchanged as fallback

Testing

• 7 new tests in MobileCoreRuntimeTests (init validation, protocol rejection, state pass-through across the bridge, brain-throws fallback) run against real JavaScriptCore.
• Full suite: 121/121 green on iPhone 17 Pro simulator.
• End-to-end (bundle fetched from a live local server, decision logged, survey rendered incl. multi-language resolution) verified on the Android twin of this PR; the iOS runtime was verified against the same built bundle in-process.

Draft — remaining scope for ENG-2900

• Opt-out flag on FormbricksConfig (disable remote code for enterprise policies).
• HTTPS enforcement is intact in this diff (POC-era relaxations were dropped before commit).

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Note

AI model used — claude-fable-5, reasoning effort unknown.

[sonarqubecloud]

Quality Gate failed

Failed conditions
58.8% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud