fix: Remove Outdated previousSessionSuccessfullyClosed Logic

[BrandonStalnaker]

Background

• We saw a customer crash in the SDK when the app went to background. The stack trace pointed to writing previousSessionSuccessfullyClosed to disk.
• That value was no longer useful (it effectively always resolved to YES), so keeping that write path added risk without real benefit.

What Has Changed

• Removed previousSessionSuccessfullyClosed logic from MPBackendController.
• Removed related constants and cleanup code tied to that value.
• Net effect: the SDK no longer performs the disk write path from the reported crash stack.

Screenshots/Video

• This code is from the initial commit of the sdk

Checklist

• I have performed a self-review of my own code.
• I have tested this locally.

Additional Notes

• Confirmed that the Android code base already didn't send this flag in its launch message

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

• Closes https://go/j/[SDKE-1101]

[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	1.82 MB	1.82 MB	+N/A
Executable Impact	896 bytes	896 bytes	+N/A
XCFramework Size	9.49 MB	9.48 MB	-8 KB

➡️ SDK size impact change is minimal.

Raw measurements

Target branch (main):

{"baseline_app_size_kb":84,"baseline_executable_size_bytes":75464,"with_sdk_app_size_kb":1944,"with_sdk_executable_size_bytes":76360,"sdk_impact_kb":1860,"sdk_executable_impact_bytes":896,"xcframework_size_kb":9720}

This PR:

{"baseline_app_size_kb":84,"baseline_executable_size_bytes":75464,"with_sdk_app_size_kb":1944,"with_sdk_executable_size_bytes":76360,"sdk_impact_kb":1860,"sdk_executable_impact_bytes":896,"xcframework_size_kb":9712}