fix(chat): Fix mentions at the beginning of captions

[nickvergessen]

Set the current message as comment content, so that the message finds mentions which are now part if the message but were not on the original comment, e.g. mentions at the beginning of captions

☑️ Resolves

• Fix Mentioning in file captions does not work when at the start of the caption #10830

🛠️ API Checklist

Before

    Then user "participant1" sees the following messages in room "public room" with 200 # FeatureContext::userSeesTheFollowingMessagesInRoom()
      | room        | actorType | actorId      | actorDisplayName         | message         | messageParameters |
      | public room | users     | participant1 | participant1-displayname | {mention-user1} | "IGNORE"          |
      Failed asserting that two arrays are equal.
      --- Expected
      +++ Actual
      @@ @@
               'actorType' => 'users'
               'actorId' => 'participant1'
               'actorDisplayName' => 'participant1-displayname'
      -        'message' => '{mention-user1}'
      +        'message' => '@participant2'
               'messageParameters' => '"IGNORE"'
           )
       )

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not possible
• 📘 API documentation in docs/ has been updated or is not required
• 🔖 Capability is added or not needed

[SystemKeeper]

Tested on web and on iOS