fix: Make SentryScope.useSpan non-blocking

[philipphofmann]

📜 Description

Instead of calling the callback of useSpan within the spanLock, get a reference to the span inside the critical sector and pass this to the callback after the critical sector closes. This way, when the callback is a blocking call, useSpan doesn't block other invocations of itself and prevents potential deadlocks.

💡 Motivation and Context

A customer reported a deadlock on macOS for which multiple threads pointed to acquiring locks to SentryScope useSpan.

💚 How did you test it?

Unit tests.

📝 Checklist

You have to check all boxes before merging:

• I reviewed the submitted code.
• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

🔮 Next steps

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (42ef6ba) 89.244% compared to head (70c017e) 89.223%.

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #3568       +/-   ##
=============================================
- Coverage   89.244%   89.223%   -0.021%     
=============================================
  Files          529       529               
  Lines        57689     57692        +3     
  Branches     20641     20636        -5     
=============================================
- Hits         51484     51475        -9     
- Misses        5292      5300        +8     
- Partials       913       917        +4     

Files	Coverage Δ
Sources/Sentry/SentryScope.m	96.791% <100.000%> (+0.017%)	⬆️
Tests/SentryTests/SentryScopeSwiftTests.swift	99.029% <100.000%> (+0.068%)	⬆️

... and 16 files with indirect coverage changes

---

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[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1225.92 ms	1242.69 ms	16.78 ms
Size	21.58 KiB	417.86 KiB	396.27 KiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
1734d1b	1200.15 ms	1214.06 ms	13.92 ms
228a909	1248.92 ms	1267.42 ms	18.50 ms
d3edf46	1213.00 ms	1227.46 ms	14.46 ms
6e342ac	1220.24 ms	1240.14 ms	19.90 ms
be2977c	1202.51 ms	1221.32 ms	18.81 ms
e778bd2	1224.66 ms	1252.16 ms	27.50 ms
e998fd0	1241.49 ms	1262.63 ms	21.14 ms
72c8d84	1238.96 ms	1247.34 ms	8.38 ms
98a8c16	1234.69 ms	1265.02 ms	30.33 ms
b9d59f7	1250.71 ms	1257.78 ms	7.06 ms

App size

Revision	Plain	With Sentry	Diff
1734d1b	21.58 KiB	418.82 KiB	397.23 KiB
228a909	20.76 KiB	425.71 KiB	404.95 KiB
d3edf46	20.76 KiB	436.65 KiB	415.89 KiB
6e342ac	20.76 KiB	436.66 KiB	415.90 KiB
be2977c	22.85 KiB	407.67 KiB	384.83 KiB
e778bd2	20.76 KiB	426.15 KiB	405.39 KiB
e998fd0	21.58 KiB	414.59 KiB	393.01 KiB
72c8d84	22.85 KiB	408.88 KiB	386.03 KiB
98a8c16	20.76 KiB	431.00 KiB	410.24 KiB
b9d59f7	22.85 KiB	405.77 KiB	382.93 KiB

[github-actions]

	Fails
🚫	Please consider adding a changelog entry for the next release.

Instructions and example for changelog

Please add an entry to CHANGELOG.md to the "Unreleased" section. Make sure the entry includes this PR's number.

Example:

## Unreleased

- Make SentryScope.useSpan non-blocking ([#3568](https://github.com/getsentry/sentry-cocoa/pull/3568))

If none of the above apply, you can opt out of this check by adding #skip-changelog to the PR description.

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