Add channel communication methods (messages, mailbox, inbox)

[artugro]

Summary

SDK methods for sending messages within a communication network and polling inbox.

New methods

Sending

• send_message(network_id, sender_id, recipient_id, content) → NetworkMessage
• send_to_mailbox(network_id, sender_id, recipient_id, content) → NetworkMessage

Receiving

• get_inbox(network_id, participant_id, limit=50) → list[NetworkMessage]
• acknowledge_messages(network_id, message_ids) → int

Messages

• list_messages(network_id, limit=100) → list[NetworkMessage]

New model

class NetworkMessage:
    id: str
    network_id: str
    sender_participant_id: str
    recipient_participant_id: str | None
    channel_type: str  # call | message | mailbox
    content: str
    status: str  # pending | delivered | read | failed
    created_at: str

Design notes

• Calls (POST /networks/{id}/call) can be added later — they're more complex (synchronous, blocking)
• Keep the API simple: client.send_message(...) not client.networks.channels.message.send(...)

Backend endpoints

• POST /networks/{id}/messages/send
• POST /networks/{id}/mailbox
• GET /networks/{id}/inbox/{pid}
• POST /networks/{id}/messages/ack
• GET /networks/{id}/messages