Added missing DSN field into the SentryEnvelopeHeade

[marandaneto]

📜 Description

Cont. #873

💡 Motivation and Context

Part of #872

💚 How did you test it?

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• I updated the docs if needed
• All tests passing
• No breaking changes

🔮 Next steps

[codecov-commenter]

Codecov Report

Base: 90.44% // Head: 90.45% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (a33faf9) compared to base (b8b9b7b).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1050      +/-   ##
==========================================
+ Coverage   90.44%   90.45%   +0.01%     
==========================================
  Files         114      114              
  Lines        3578     3584       +6     
==========================================
+ Hits         3236     3242       +6     
  Misses        342      342              

Impacted Files	Coverage Δ
dart/lib/src/sentry_client.dart	93.52% <100.00%> (+0.14%)	⬆️
dart/lib/src/sentry_envelope.dart	100.00% <100.00%> (ø)
dart/lib/src/sentry_envelope_header.dart	100.00% <100.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[krystofwoldrich]

Looks good to me.