chore: migrate iOS Rokt integration to SDK 9

[denischilik]

Background

This PR updates the Flutter plugin iOS integration to the SDK 9 contract surface so the plugin continues to compile and emit Rokt events after dependency upgrades.
This PR is intentionally scoped to migration and compatibility only.

What Has Changed

• Updated iOS pod dependencies to SDK 9 (mParticle-Apple-SDK, mParticle-Apple-SDK-ObjC) and added RoktContracts
• Raised iOS deployment target to 15.6 for plugin/example compatibility with SDK 9
• Migrated iOS plugin bridge types from legacy MPRokt* APIs to SDK 9 Rokt* contract types
• Added event payload mapping for new shoppable event types in RoktEventHandler (CartItemInstantPurchaseInitiated, CartItemInstantPurchaseFailure, InstantPurchaseDismissal, CartItemDevicePay)

Screenshots/Video

N/A

Checklist

• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.
• I have added tests that prove my fix is effective or that my feature works.
• I have tested this locally.

Additional Notes

Shoppable ads API surface changes are not included here and will be added in a follow-up PR.

[cursor]

PR Summary

Medium Risk
Moderate risk due to dependency upgrades (mParticle SDK 9) and contract/type migrations that can break build/runtime event bridging, plus raising the iOS deployment target to 15.6 which drops support for older devices.

Overview
Updates the iOS side of the Flutter plugin and example app to mParticle SDK 9 by switching from legacy MPRokt* APIs to RoktContracts types (e.g., RoktEmbeddedView, RoktEvent, RoktConfig).

Raises the iOS deployment target to 15.6 across the example project and podspec, and updates pod dependencies to the SDK 9 split (mParticle-Apple-SDK + mParticle-Apple-SDK-ObjC) while adding RoktContracts.

Adjusts Rokt placement selection and event bridging: removes the old callback object in favor of the new selectPlacements event callback, updates embedded size-change handling, and expands event-to-Flutter payload mapping for additional commerce/shoppable events.

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