[v9]: remove options.autoAppStart and setAppStartEnd

[buenaflor]

📜 Description

v9 task

💡 Motivation and Context

SDK consistency, and these APIs just make it harder to maintain it.
Users can already disable the integration if they wish to without the autoAppStart flag

💚 How did you test it?

Existing tests are enough since we remove functionality

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• No new PII added or SDK only sends newly added PII if sendDefaultPii is enabled
• I updated the docs if needed
• All tests passing
• No breaking changes

🔮 Next steps

[github-actions]

🚨 Detected changes in high risk code 🚨

High-risk code has higher potential to break the SDK and may be hard to test. To prevent severe bugs, apply the rollout process for releasing such changes and be extra careful when changing and reviewing these files:

• flutter/lib/src/integrations/native_app_start_integration.dart

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.62%. Comparing base (3e1c2c9) to head (dd0799c).
Report is 1 commits behind head on v9.

Additional details and impacted files

@@            Coverage Diff             @@
##               v9    #2680      +/-   ##
==========================================
+ Coverage   89.06%   92.62%   +3.55%     
==========================================
  Files         261       89     -172     
  Lines        8847     2873    -5974     
==========================================
- Hits         7880     2661    -5219     
+ Misses        967      212     -755     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.