fix(otel): only set Scope.Transaction in SentrySpanProcessor when null

[tsushanth]

Fixes #5314.

Problem

SentrySpanProcessor.OnStart calls ConfigureScope unconditionally:

_hub.ConfigureScope(static (scope, transaction) => scope.Transaction = transaction, transaction);

When two root OTel activities overlap and neither is a child of the other, this silently overwrites whatever transaction was already on the scope with the new one. As the issue notes, the intuitive default is to not overwrite an existing transaction.

Fix

Add a null-check guard inside the ConfigureScope lambda:

_hub.ConfigureScope(static (scope, transaction) =>
{
    if (scope.Transaction is null)
    {
        scope.Transaction = transaction;
    }
}, transaction);

Scope.Transaction is only set when it is currently null; an already-present transaction is left unchanged.

Test

Added OnStart_WithExistingTransactionOnScope_DoesNotOverwriteExistingTransaction to SentrySpanProcessorTests: sets a transaction on the scope, starts a new root activity, calls OnStart, and asserts the scope still holds the original transaction.

This is intentionally not bundled with #2399 (the AutoSetScopeTransactions opt-in), per the issue's notes.

[Flash0ver]

Thank you very much, @tsushanth, for your contribution to the Sentry .NET SDK!
I left some comments about the test-case, and commented on the comments 😉

[tsushanth]

Thanks for the detailed review @Flash0ver! Updated:

• Source comment: shortened to a two-liner with issue link
• Test: rewritten to use ScopeManager/Scope directly (same pattern as OnStart_SampledWithoutParentSpanId_StartsNewTransaction), so it actually exercises CreateRootSpan. First activity sets scope.Transaction via the processor; second root activity is the act; assertion checks scope.Transaction is still the first
• Types: removed ITransaction (doesn't exist here); scope.Transaction is tested via BeSameAs on the captured reference
• Inline comments: removed the narrative comments, kept only Arrange/Act/Assert markers

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

Want reviews to match your repository better? Bugbot Learning can learn team-specific rules from PR activity. A team admin can enable Learning in the Cursor dashboard.

^{Reviewed by Cursor Bugbot for commit 8a20eba. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.20%. Comparing base (04a9854) to head (49584fd).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5337      +/-   ##
==========================================
+ Coverage   74.16%   74.20%   +0.03%     
==========================================
  Files         508      509       +1     
  Lines       18353    18389      +36     
  Branches     3586     3601      +15     
==========================================
+ Hits        13612    13645      +33     
- Misses       3869     3870       +1     
- Partials      872      874       +2     

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[tsushanth]

Sorry about the delay — was pushing to the wrong local branch name. Commits are now on the correct branch: ??= operator in the source, and the test rewritten to use the real Hub (not a mock IInternalScopeManager) so ConfigureScope actually executes the null guard.

[Flash0ver]

thanks ... I'll take another look in the morning (CEST)

[Flash0ver]

One final suggestion ... to make the test actually cover the branch where scope.Transaction is already set AND _hub.ConfigureScope is invoked.
(I took the liberty to apply the change and re-run the checks/workflows)