fix(addToMessageList): prevent messages shifting when adding reactions

[arnautov-anton]

This regression has been introduced with the refactor of the function in question in #1330. Tests have been built on previous version of this function and applied to a new version with a fix. This PR also fixes a test case in threads.test.ts.

[github-actions]

Size Change: +22 B (+0.01%)

Total Size: 427 kB

Filename	Size	Change
dist/browser.es.js	92.6 kB	+6 B (+0.01%)
dist/browser.full-bundle.min.js	54.1 kB	-3 B (-0.01%)
dist/browser.js	93.8 kB	+7 B (+0.01%)
dist/index.es.js	92.7 kB	+6 B (+0.01%)
dist/index.js	93.8 kB	+6 B (+0.01%)

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