Send typing indicator

Show a typing indicator in a conversation. Platform support:

• Facebook Messenger: Shows "Page is typing..." for 20 seconds
• Instagram: Shows "typing..." to the recipient (works for both Instagram Login and Facebook Login accounts). The recipient must be signed in to Instagram to see it.
• Telegram: Shows "Bot is typing..." for 5 seconds
• WhatsApp: Shows "typing..." for up to 25 seconds. Requires a recent inbound message in the conversation (Meta references the inbound message id) and also marks that message as read as a side-effect.
• All others: Returns 200 but no-op (platform doesn't support it)

Typing indicators are best-effort. The endpoint always returns 200 even if the platform call fails; success reports whether a typing indicator was actually sent to the platform (false on unsupported platforms or when the platform call failed).