feat: support Telegram Bot API 9.6 (April 3, 2026)

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Summary

Track implementation of Telegram Bot API 9.6 (April 3, 2026) in go-telegram/bot: models, methods_params.go, methods.go, multipart/build_request_form where needed, tests, README and CHANGELOG.

References

• Changelog: https://core.telegram.org/bots/api#april-3-2026

Scope (high level)

Managed bots

• New / updated types and fields: e.g. can_manage_bots on User, KeyboardButtonRequestManagedBot, PreparedKeyboardButton, ManagedBotCreated, ManagedBotUpdated, updates on Message / Update.
• New methods: getManagedBotToken, replaceManagedBotToken, savePreparedKeyboardButton.
• Web App: WebApp.requestChat, deep link https://t.me/newbot/... support surface in docs/examples if applicable.

Polls (expect breaking API alignment)

• Replace correct_option_id with correct_option_ids in Poll and sendPoll params; quiz multiple correct answers; new flags/parameters (allows_revoting, shuffle_options, allow_adding_options, hide_results_until_closes, descriptions, PollOption additions, PollOptionAdded / PollOptionDeleted, ReplyParameters.poll_option_id, Message.reply_to_poll_option_id, etc.).
• Increased max poll close time; document behavior changes.

Other

• Broader use of date_time entities where noted in the official changelog (checklist, quotes, gifts, etc.).

Acceptance criteria

• All new/changed types and methods from the 9.6 changelog are represented consistently with existing library patterns (see e.g. merged PR for Bot API 9.5).
• go test ./... passes; README states supported Bot API version after 9.6.
• CHANGELOG entry lists user-visible additions and breaking changes (especially Poll / correct_option_ids).

Notes

• Coordinate with other open work (e.g. chat id int → int64 in params — issue SendChecklistParams.ChatID and some other params chat id field has wrong type int instead of int64 #273) to avoid duplicate or conflicting PRs.