Add a Telegram adapter

[TroyHernandez]

Fourth adapter, the one DESIGN.md calls "nearly free once the poll contract exists". chat_telegram() puts the Bot API behind the contract with HTTP delegated to httr, which was already in Suggests, so the dependency footprint does not change.

• Receive is getUpdates long polling. One call returns every update Telegram is holding for the bot across every chat it is in, so the cursor is a single update offset rather than one per channel. The first poll returns whatever Telegram is still holding (up to 24h), which is the mail that arrived while the bot was down rather than history, so it comes out as ordinary traffic. allowed_updates asks for messages, channel posts, and reaction events; edits are not requested.
• Sends render the contract's markdown to Telegram's HTML subset (MarkdownV2 wants a dozen prose characters escaped), carry thread as message_thread_id and reply_to as reply_parameters, send each file as its own sendDocument ahead of the text so ids come back in the order Matrix uses, and honour notify = FALSE. A supplied rich fragment is sent as HTML.
• Edits, emoji reactions (an empty key clears), reaction events as chat_reaction records (additions only; a removal has no shape in the contract), typing, getChat info, leaveChat, getMe identity, setMyName, and attachment fetch through getFile are wired. chat_addressed() reads @username from the body, treats a private chat and a reply to the bot as addressed, and still honours declared text_mention entities.
• Capabilities are honest about the Bot API: no history, member list, chat list, read markers, joining, or creating. Reaction events reach a bot in a group only where it is an administrator, and the roxygen says so.
• Every id goes through sprintf("%.0f") rather than as.character(): R writes the double 100000 as "1e+05", and every id Telegram sends is a number to its JSON parser.

Tests: every verb runs through a .api seam that asserts on the wire shape each Bot API method receives (form-encoded, NULLs dropped, logicals lowercase), and httr's formals are pinned for drift. The real HTTP layer was probed once against api.telegram.org with a fake token: the 401 surfaces as "Telegram refused getMe: Unauthorized" through the parsed body. 1363 tests pass under R CMD check; the one WARNING (chat_edit()'s undocumented rich) is pre-existing on main and gets its own PR.