refactor: MPCommerceEvent pure objc

[nickolas-dimitrakas]

Background

• To aid in migrating to source based distribution we need to migrate from ObjC++ code to plain ObjC

What Has Changed

• MPCommerceEvent

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.

Reference Issue (For employees only. Ignore if you are an outside contributor)

• Closes https://go/j/[SDKE-723]

[github-actions]

📦 SDK Size Impact Report

Measures how much the SDK adds to an app's size (with-SDK minus without-SDK).

Metric	Target Branch	This PR	Change
App Bundle Impact	1.67 MB	1.65 MB	-16 KB
Executable Impact	896 bytes	896 bytes	+N/A
XCFramework Size	9.12 MB	9.05 MB	-72 KB

➡️ SDK size impact change is minimal.

Raw measurements

Target branch (workstation/9.0-Release):

{"baseline_app_size_kb":84,"baseline_executable_size_bytes":75464,"with_sdk_app_size_kb":1792,"with_sdk_executable_size_bytes":76360,"sdk_impact_kb":1708,"sdk_executable_impact_bytes":896,"xcframework_size_kb":9336}

This PR:

{"baseline_app_size_kb":84,"baseline_executable_size_bytes":75464,"with_sdk_app_size_kb":1776,"with_sdk_executable_size_bytes":76360,"sdk_impact_kb":1692,"sdk_executable_impact_bytes":896,"xcframework_size_kb":9264}