In Telegram forum-enabled groups, the General topic does not include
message_thread_id in incoming messages (it is None). This caused:
1. Messages in General losing thread context — replies went to wrong place
2. Typing indicator failing because thread_id=1 was rejected by Telegram
Fix: synthesize thread_id="1" for forum groups when message_thread_id
is None, then handle it correctly per operation:
- send: omit message_thread_id (Telegram rejects thread_id=1 for sends)
- typing: pass thread_id=1, retry without it on "thread not found"
Also centralizes thread_id extraction into _metadata_thread_id() across
all send methods (send, send_voice, send_image, send_document, send_video,
send_animation, send_photo), replacing ~10 duplicate patterns.
Salvaged from PR #7892 by @corazzione.
Closes#7877, closes#7519.
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