Adds the getChat method to the HTTP Bot API gateway. Numeric chat_id only (no
@username). Resolution goes through the shared peer resolvers:
- user: ByID; unknown -> CHAT_NOT_FOUND
- channel/supergroup: ResolveChannel, so a public one resolves even when the bot
is not a member (projected as a preview); a private one the bot cannot access
-> CHAT_NOT_FOUND, a banned bot likewise
The Chat projection returns id (bot-api encoded), type ("channel" for a
broadcast, "supergroup" for a megagroup, "private" for a user), title, username,
first/last name, description, is_forum, and the scam/fake/verified flags.
