Fix: Breadcrumbs "Concurrent Modification"

[denrase]

📜 Description

Closes #941

💚 How did you test it?

Ran unit tests.

📝 Checklist

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

[codecov-commenter]

Codecov Report

Merging #948 (a67dbad) into main (5a35e4a) will increase coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #948      +/-   ##
==========================================
+ Coverage   89.81%   89.94%   +0.12%     
==========================================
  Files         104        9      -95     
  Lines        3211      169    -3042     
==========================================
- Hits         2884      152    -2732     
+ Misses        327       17     -310     

Impacted Files	Coverage Δ
dart/lib/src/scope.dart
dart/lib/src/protocol/sentry_request.dart
dart/lib/src/protocol/sentry_id.dart
dart/lib/src/sentry_envelope.dart
...ib/src/environment/_web_environment_variables.dart
dart/lib/src/protocol/sentry_gpu.dart
...event_processor/deduplication_event_processor.dart
dart/lib/src/protocol/sentry_stack_trace.dart
dart/lib/src/sentry_traces_sampler.dart
dart/lib/src/http_client/tracing_client.dart
... and 85 more

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