Fix display of amendment with formatting

This is the new description from: https://github.com/OpenSlides/openslides-client/issues/4452

Description:
If you have formatting in the motion anf Umlaute then changeing part of the formating and more with the amendment breaks the amendment.

Broken formatting:

• Bold + Italic
• Bold + underlined
• Bold + strikethrough
• Bold + Italic + underlined
• Bold + Italic + strikethrough
• Bold+italic+underlined+strikethrough
• Italic + underlined
• italic + strikethrough
• italic + underlined + strike
• underlied + strikethrough
• all combinations with text color -> these can also get heir own Issue if the others above are fixed
• all combinations with background color -> these can also get heir own Issue if the others above are fixed

the motion text:

<p>Und es war ihnen wie eine <strong>
<em>Bestätigung</em>
</strong> ihrer neuen Träume und guten Absichten, als am Ziele ihrer Fahrt die Tochter als erste sich erhob und ihren jungen <strong>
<em>Körper</em>
</strong> dehnte. motion with two paragraphs. Add in the second paragraph a bold formatting of one word in the first and in the second senence.</p>

Example amendment test for bold and italic (line 5-7 was changed):

<p>Und es war ihnen wie eine <strong>
<em>Bestätigung</em>
</strong> ihrer neuen Träume und guten Absichten, als am Ziele ihrer Fahrt die Tochter als erste sich erhob und ihren jungen alten <strong>
<em>Körpergehülle</em>
</strong> dehnte. motion with two paragraphs. Add in the second paragraph a bold formatting of one word in the first and in the second senence.</p>

Use the two HTML snippets in the issue as reproduction inputs and compare amendment output for the listed formatting combinations, including umlauts. Locate the client-side amendment and formatting entry points, then add coverage showing that formatting is preserved after edits and verify the listed cases.