[draft] fix(web): don't create runZoneGuarded on web if it already exists

[buenaflor]

📜 Description

If the sentry init is already set up in a zone, don't create another one internally

We can still keep the onError callback and propagate the user's onError on top of it so we can keep the original behaviour

💡 Motivation and Context

Fixes #1943

💚 How did you test it?

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• No new PII added or SDK only sends newly added PII if sendDefaultPii is enabled
• I updated the docs if needed
• All tests passing
• No breaking changes

🔮 Next steps

[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

- don't create runZoneGuarded on web if it already exists ([#2088](https://github.com/getsentry/sentry-dart/pull/2088))

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

Generated by 🚫 dangerJS against 34ea139

[codecov]

Codecov Report

Attention: Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.

Project coverage is 95.09%. Comparing base (e6b16cd) to head (34ea139).

Files	Patch %	Lines
flutter/lib/src/sentry_flutter.dart	57.14%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2088      +/-   ##
==========================================
- Coverage   95.25%   95.09%   -0.17%     
==========================================
  Files          54       54              
  Lines        1791     1793       +2     
==========================================
- Hits         1706     1705       -1     
- Misses         85       88       +3     

☔ View full report in Codecov by Sentry.
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[github-actions]

Android Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	393.15 ms	462.71 ms	69.57 ms
Size	6.35 MiB	7.33 MiB	1005.84 KiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
bffc2c5	348.00 ms	399.89 ms	51.89 ms
62de927	313.81 ms	358.15 ms	44.34 ms
8cb6557	321.20 ms	370.46 ms	49.26 ms
ca7f531	395.69 ms	497.82 ms	102.12 ms
24b6e60	440.64 ms	557.96 ms	117.32 ms
895becc	326.94 ms	376.02 ms	49.08 ms
be08ed1	361.37 ms	414.84 ms	53.47 ms
613760b	373.42 ms	399.33 ms	25.92 ms
2d74010	400.42 ms	466.50 ms	66.08 ms
68677de	364.41 ms	415.61 ms	51.20 ms

App size

Revision	Plain	With Sentry	Diff
bffc2c5	6.34 MiB	7.28 MiB	966.31 KiB
62de927	6.15 MiB	7.11 MiB	981.78 KiB
8cb6557	6.06 MiB	7.03 MiB	997.01 KiB
ca7f531	6.33 MiB	7.26 MiB	949.75 KiB
24b6e60	6.33 MiB	7.26 MiB	950.14 KiB
895becc	6.06 MiB	7.03 MiB	997.23 KiB
be08ed1	6.33 MiB	7.26 MiB	946.42 KiB
613760b	5.94 MiB	6.92 MiB	1005.98 KiB
2d74010	6.33 MiB	7.26 MiB	943.41 KiB
68677de	6.06 MiB	7.10 MiB	1.04 MiB

[github-actions]

iOS Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1245.20 ms	1262.00 ms	16.80 ms
Size	8.33 MiB	9.54 MiB	1.22 MiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
e82709a	1249.22 ms	1271.35 ms	22.12 ms
0a23f98	1252.98 ms	1276.76 ms	23.78 ms
6325c3b	1266.52 ms	1291.06 ms	24.54 ms
6078ddc	1207.84 ms	1224.10 ms	16.27 ms
24f71aa	1267.47 ms	1272.00 ms	4.53 ms
1c6eb5b	1277.85 ms	1285.71 ms	7.86 ms
8609bd8	1267.16 ms	1291.39 ms	24.22 ms
0a82a1e	1233.56 ms	1244.56 ms	11.00 ms
2d74010	1264.45 ms	1268.42 ms	3.97 ms
72dfc83	1262.50 ms	1289.75 ms	27.25 ms

App size

Revision	Plain	With Sentry	Diff
e82709a	8.33 MiB	9.40 MiB	1.07 MiB
0a23f98	8.10 MiB	9.18 MiB	1.08 MiB
6325c3b	8.16 MiB	9.17 MiB	1.01 MiB
6078ddc	8.33 MiB	9.40 MiB	1.07 MiB
24f71aa	8.10 MiB	9.16 MiB	1.07 MiB
1c6eb5b	8.15 MiB	9.12 MiB	986.27 KiB
8609bd8	8.28 MiB	9.34 MiB	1.06 MiB
0a82a1e	8.29 MiB	9.37 MiB	1.08 MiB
2d74010	8.32 MiB	9.38 MiB	1.05 MiB
72dfc83	8.15 MiB	9.12 MiB	987.30 KiB