Export plaintext message content in JSON text field

[shaunnope]

Related Problem
As of v4.8.1, when exporting chats as JSON, the text fields of messages has two return types:

1. string if message is entirely in plain text, with no message entities
2. array of mixed types otherwise (string for plain text, object for styled text)

This was first identified in #25000.

Consider the following message:

Lorem ipsum dolor sit amet, consectetur adipiscing elit.
Morbi lobortis lacus eget leo iaculis maximus.

The text field of the resulting JSON export for the message is

"text": [
    "Lorem ipsum dolor sit",
    {
     "type": "bold",
     "text": " amet, consectetur"
    },
    " ",
    {
     "type": "italic",
     "text": "adipiscing elit"
    },
    ".\n",
    {
     "type": "code",
     "text": "Morbi lobortis"
    },
    " lacus eget leo iaculis maximus."
   ],

Proposed Solution
Concatenate the plaintext content for all message entities, producing a single string for the text field.

For example, the text and text_entity fields for message above would be exported as

{
   "text": "Lorem ipsum dolor sit amet, consectetur adipiscing elit.\nMorbi lobortis lacus eget leo iaculis maximus.",
   "text_entities": [
    {
     "type": "plain",
     "text": "Lorem ipsum dolor sit"
    },
    {
     "type": "bold",
     "text": " amet, consectetur"
    },
    {
     "type": "plain",
     "text": " "
    },
    {
     "type": "italic",
     "text": "adipiscing elit"
    },
    {
     "type": "plain",
     "text": ".\n"
    },
    {
     "type": "code",
     "text": "Morbi lobortis"
    },
    {
     "type": "plain",
     "text": " lacus eget leo iaculis maximus."
    }
   ]
  }

This behaviour is also in keeping with the Telegram API and Bot API, where Message objects have message and text fields respectively for storing the plaintext content of a message.

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