Allow users to filter message searches by channel sender in groups

[TheWug]

Users can send messages as channels in any supergroup which either has a public username or is linked to a channel. Previously it was not possible to filter by sender for messages sent on behalf of a channel, and now it is. In order to avoid confusion, you must search by the channel's exact username, and the group must permit sending messages as channels.

It is worth considering whether chat administrators should always be able to search for messages by any channel, regardless of group or channel settings, to ensure they can handle adversarial situations, but this pull request addresses only the simple case.

closes #25105

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✅ TheWug
❌ Wug

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