How can I recover accidentally deleted text message thread without resetting my phone?

If you have another device signed into iCloud, you can see if the messages are still there or if they got deleted on all devices. Otherwise, there isn't a way to recover deleted messages without resetting/restoring an iCloud backup.

If you had an iCloud backup when your messages were there, you can restore the backup: Restore your iPhone, iPad, or iPod touch from a backup - Apple Support. Otherwise, unfortunately, you cannot recover them.