feat(telegram/plugin): port guest mode + buffer fixes to adapter.py

[elphamale]

Upstream commit 5600105 moved gateway/platforms/telegram.py → plugins/platforms/telegram/adapter.py as part of the plugin system migration. PRs #43049, #49116, and #49186 all targeted the old path and now conflict.

This PR is the consolidated replacement, applying the same changes to the new file:

Changes

From #43049 (guest mode delivery)

• State vars: _pending_guest_queries, _guest_only_chats, _guest_reply_buffer in __init__
• send() buffer block: routes guest-chat calls to the reply buffer instead of rejected sendMessage
• TypeHandler registration for guest_message updates (Bot API 10.0 raw field)
• _handle_guest_message_update() handler — parses payload, stores guest_query_id, routes through normal pipeline
• on_processing_complete() flush — fires answerGuestQuery with the buffered response
• Media-method guards (send_voice, send_image_file, send_document, send_video, send_image) return a descriptive error for guest chats where the bot can't upload files

From #49116 (sender_chat identity)

• _build_message_event(): when from_user is None (user posting as a channel), fall back to sender_chat.id / sender_chat.title for user identity so auth checks match and session keys are stable

From #49186 (buffer quality)

• Drop tool-progress blocks from the guest buffer (expect_edits/notify metadata flag detection)
• Strip the streaming cursor ▉ before buffering
• startswith-accumulation: cumulative streaming updates replace the buffer; overflow chunks append

Supersedes #43049, #49116, #49186.

[elphamale]

Closing as superseded by #56476/#56477.

This PR was a mechanical port of #43049/#49116/#49186's logic from the old gateway/platforms/telegram.py into plugins/platforms/telegram/adapter.py. All three of those are now closed as superseded by the same rebuild, so this port is superseded wholesale along with them.