feat: Add SentryOptions.AutoSetScopeTransactions to auto-store transactions on the scope

[jamescrosswell]

Closes #2399
Closes #5313

What

This is a QOL improvement that adds an opt-in SentryOptions.AutoSetScopeTransactions option (defaults to false). When enabled, StartTransaction automatically stores the started transaction on the current scope, so APIs like SentrySdk.GetSpan() / StartChild() work without the user having to manually call SentrySdk.ConfigureScope(scope => scope.Transaction = transaction).

Since SentryOptions.AutoSetScopeTransactions defaults to false this is opt-in and non-breaking. We could potentially change the default in future major releases though.

Per-call override (internal)

StartSpan/StartTransaction gain an internal bool? autoSetScopeTransaction override that wins over the global option, so integrations can control this independently of the user's setting. The MAUI CommunityToolkit binder uses it to ensure we don't store the transaction on the (global, AsyncLocal-backed) scope even when a user enables the AutoSetScopeTransactions option globally.

Note

This changes the MAUI CommunityToolkit binder to finally match its own comment - see #5313

[codecov]

Codecov Report

❌ Patch coverage is 91.17647% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.21%. Comparing base (f01f273) to head (e10ede3).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Sentry/Scope.cs	85.71%	0 Missing and 2 partials ⚠️
...olkit.Mvvm/MauiCommunityToolkitMvvmEventsBinder.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5317      +/-   ##
==========================================
+ Coverage   74.16%   74.21%   +0.04%     
==========================================
  Files         508      508              
  Lines       18353    18376      +23     
  Branches     3586     3589       +3     
==========================================
+ Hits        13612    13637      +25     
+ Misses       3869     3867       -2     
  Partials      872      872              

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[bitsandfoxes]

I'm not entirely convinced of this option. This requires such a detailed mental model of the way the SDK handles Spans and Transactions that I don't think this would actually help an actual user.
And on top there is the move to be span first that'd make it obsolete. And I know we've been talking about making transactions obsolete for over a year but now it's happening for real. 😅

[jamescrosswell]

And I know we've been talking about making transactions obsolete for over a year but now it's happening for real.

Probably more like 2-3 years but yes, it does feel like we're getting closer.

I guess we close as won't do for now then and rely on the move to span first to resolve this. We may still require some notion of Scope.ActiveSpan in that world though as we still have the problem of 'which span should be the parent of this node'... semi-solved in OTEL (Activity.Current) but not in the Sentry tracing APIs without the use of scope.