fix: Reads UI Windows in the main thread while fetching view hierarchy

[brustolin]

📜 Description

Reads UI Windows in the main thread while fetching view hierarchy

💡 Motivation and Context

Closes #2586

💚 How did you test it?

Unit test

📝 Checklist

• I reviewed the submitted code
• I added tests to verify the changes
• I updated the docs if needed
• Review from the native team if needed
• No breaking changes

🔮 Next steps

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1243.45 ms	1256.88 ms	13.43 ms
Size	20.76 KiB	419.70 KiB	398.94 KiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
3f1be0f	1208.12 ms	1225.72 ms	17.60 ms
c6504da	1232.06 ms	1243.28 ms	11.22 ms
cb2eefe	1222.04 ms	1243.04 ms	21.00 ms
369222e	1232.14 ms	1258.90 ms	26.76 ms
621ba9b	1190.66 ms	1230.84 ms	40.18 ms
56ec5d0	1236.65 ms	1261.90 ms	25.25 ms
3a31fc9	1237.35 ms	1249.02 ms	11.67 ms
feba2be	1246.67 ms	1254.64 ms	7.97 ms
56ec5d0	1194.39 ms	1236.94 ms	42.55 ms
4c00f8c	1231.62 ms	1237.76 ms	6.14 ms

App size

Revision	Plain	With Sentry	Diff
3f1be0f	20.76 KiB	414.44 KiB	393.69 KiB
c6504da	20.76 KiB	414.44 KiB	393.69 KiB
cb2eefe	20.76 KiB	419.62 KiB	398.86 KiB
369222e	20.76 KiB	419.67 KiB	398.91 KiB
621ba9b	20.76 KiB	414.45 KiB	393.69 KiB
56ec5d0	20.76 KiB	414.44 KiB	393.69 KiB
3a31fc9	20.76 KiB	414.45 KiB	393.69 KiB
feba2be	20.76 KiB	414.45 KiB	393.69 KiB
56ec5d0	20.76 KiB	414.44 KiB	393.68 KiB
4c00f8c	20.76 KiB	419.62 KiB	398.86 KiB

Previous results on branch: fix/scenes-main-thread

Startup times

Revision	Plain	With Sentry	Diff
d67ed1b	1210.39 ms	1224.94 ms	14.55 ms

App size

Revision	Plain	With Sentry	Diff
d67ed1b	20.76 KiB	419.62 KiB	398.86 KiB

[philipphofmann]

Thank you @brustolin 👏

[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

- Reads UI Windows in the main thread while fetching view hierarchy ([#2629](https://github.com/getsentry/sentry-cocoa/pull/2629))

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