fix ios player not detecting view change when same flow reloads

[nancywu1]

Problem:
If new flow is returned from flow manager with identical flow (just different data) we hit this guard in SwiftuiPlayer

the context of the previous flow doesnt get cleared because no new view change is detected causing it to remain in End state using the stale cached player so buttons on screen dont work (transitions)

Solution:
instead of onAppear we use onChange to detect changes in loading state, this ensures unload() is called regardless of whether SwiftUI considers the view "new", which sets player = nil and allows the caching guard to pass on the next load().

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[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.81%. Comparing base (e9286b2) to head (675191b).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
.../swiftui/Sources/ManagedPlayer/ManagedPlayer.swift	62.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #796      +/-   ##
==========================================
- Coverage   85.82%   85.81%   -0.01%     
==========================================
  Files         505      505              
  Lines       22814    22823       +9     
  Branches     2657     2657              
==========================================
+ Hits        19579    19586       +7     
- Misses       2907     2909       +2     
  Partials      328      328              

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