feat: add upload convenience methods, new permissions, and godoc exam…

…ples (v0.4.0)

Add UploadPhotoFromFile, UploadPhotoFromURL, UploadMediaFromFile,
UploadMediaFromURL with security hardening: URL scheme validation
(http/https only), 50 MB download limit, Content-Disposition filename
sanitization, and proper timeout propagation via ensureTimeout.

Introduce ErrFetch error kind for external URL and local file failures,
separating them from ErrAPI (Max Bot API errors) and ErrNetwork.

Add 5 new ChatAdminPermission constants documented on dev.max.ru and
8 example tests for godoc documentation.

feat: add ContactAttachmentPayload.Phone() to extract phone from vCard

feat: add open_app button, 5 new update types, and GoDoc field descri…

…ptions

- NewOpenAppButton(text, webApp) constructor and Button.WebApp field
- BotStoppedUpdate, DialogMutedUpdate, DialogUnmutedUpdate,
  DialogClearedUpdate, DialogRemovedUpdate with tests
- GoDoc comments on User, Chat, Message, NewMessageBody, ChatMember,
  Update and other core structs from dev.max.ru documentation
- Update types tables in guide.md, guide-ru.md
- Fix README Quick Start (add package main, func main, indentation)
- Add NewOpenAppButton to README, guide.md, guide-ru.md

fix: buffer multipart upload body to include Content-Length header

Max Bot API requires Content-Length for file uploads, returning HTTP 406
"File size is unknown" when streaming via io.Pipe. Replaced pipe-based
streaming with in-memory buffering via bytes.Buffer.

feat: add Optional[T] generics, typed attachment parsing, and multipl…

…e bug fixes

Add Optional[T] with three-state JSON semantics (unset/zero/value) replacing
*string/*bool pointers in request types. Add Attachment interface and
MessageBody.ParseAttachments() for type-safe attachment handling with all 11
attachment types. Add type-safe button constructors for inline keyboards.

Fix Optional.UnmarshalJSON treating JSON null as set instead of unset,
nil body panic in SendMessage/SendMessageToUser, goroutine leak in doUpload
on network errors, and missing default timeout in doUpload. Fix long polling
timeout racing and JSON serialization of empty attachment slices.

feat: implement full Max Bot API client

Idiomatic Go HTTP client with zero external dependencies covering
all Max Bot API endpoints: messages, chats, bots, uploads, webhooks,
and long polling. Includes structured error handling, context support,
functional options, and comprehensive test suite (55 tests).