Add support for messageRichMessage (tables, lists, code blocks, etc)

[gabrieltomasin]

Problem

Telegram recently introduced rich messages (messageRichMessage), which support advanced formatting like tables, nested lists, code blocks, collapsible sections, block quotes, and more. These are sent by bots and some clients that use the new rich message API.

Currently, nchat does not handle this message type — these messages fall through to the default else branch in TdMessageContentConvert() and are displayed as: [UnknownMessage -1243148892]

This means any message containing a table, structured list, or other rich content is completely unreadable in nchat.

Solution

This PR adds three private helper functions to TgChat::Impl that recursively convert rich message content to plain text suitable for terminal display:

GetRichText()

Recursively extracts plain text from any RichText variant (bold, italic, strikethrough, links, mentions, custom emoji, etc.), preserving the visible text content while discarding formatting markup.

AppendPageBlock()

Renders PageBlock elements as terminal-friendly plain text:

Block Type	Rendering
pageBlockTable	ASCII-aligned with +---+ borders, using wcswidth for correct column sizing with UTF-8
pageBlockList	Bullets (-) with indentation for nested items
pageBlockPreformatted	Preserved as-is with indentation
pageBlockBlockQuote	> prefix
pageBlockDetails	Header with > (collapsed) or v (open) marker
pageBlockDivider	---
Headers/subheaders/paragraphs	Plain text
Collage/slideshow/cover	Recursed into sub-blocks

GetRichMessageText()

Iterates over all PageBlocks in a richMessage and assembles the final text string.

UTF-8 display width

Table column widths are calculated using StrUtil::WStringWidth() (which calls wcswidth) rather than std::string::size(), so accented characters and emoji are sized correctly for terminal alignment.

Testing

Built and tested locally against rich messages sent via Telegram bot API. Tables, lists, code blocks, and quotes render correctly in the terminal.

Before:
[UnknownMessage -1243148892]

After (example table):

+----------+---------+---------+
| Col A    | Col B   | Col C   |
+----------+---------+---------+
| Value 1  | Value 2 | Value 3 |
| Value 4  | Value 5 | Value 6 |
+----------+---------+---------+