feat: support sending EventMessage (group events)

[NeritonDias]

Summary

WhatsApp group events (EventMessage) were effectively receive-only in whatsmeow — there was no way to create/send one. This adds the two missing pieces so client.SendMessage with an EventMessage actually renders as an event in the WhatsApp clients.

Problem

When sending a &waE2E.Message{EventMessage: ...}:

• getTypeFromMessage had no case for EventMessage, so the stanza went out with type="text". The server ACKs it, but clients silently discard it — it never renders as an event.
• The <meta event_type="creation"> child node that the protocol expects for event creation was missing (the same way polls require <meta polltype="creation">).

Sending type="event" without the meta node makes the server reject the stanza with error 479; with both, the event is created and renders correctly.

Changes (send.go)

1. getTypeFromMessage: return "event" for msg.EventMessage.
2. Append a <meta event_type="creation"> child node for event messages, mirroring the existing poll <meta polltype="creation"> handling.

Notes

• The caller must set Message.MessageContextInfo.MessageSecret (32 random bytes) so the event's responses (going/not_going EncEventResponseMessage) decrypt — same pattern as BuildPollCreation.
• Verified end-to-end against a real group: events render with title, description, location, start/end time, reminder and mentions; the going/not-going responses decrypt correctly.
• Enables the POST /send/event endpoint in evolution-go.

Summary by Sourcery

Support sending WhatsApp group EventMessage stanzas by treating them as event-type messages and including required creation metadata.

New Features:

• Allow EventMessage-based group events to be sent via client.SendMessage so they render as events in WhatsApp clients.

Bug Fixes:

• Ensure EventMessage stanzas use the correct "event" type instead of "text" to prevent clients from discarding them.
• Add required event creation meta node to avoid server errors and enable successful event creation.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Adds full support for sending WhatsApp group EventMessage stanzas by classifying them as type="event" and attaching the required node, so client.SendMessage with an EventMessage renders correctly as a group event in WhatsApp clients.

File-Level Changes

Change	Details	Files
Classify EventMessage messages as type="event" in stanza generation so they’re not treated as text.	Extend message type resolution to detect non-nil EventMessage fields. Return the stanza type string "event" when an EventMessage is present, keeping existing handling for reactions, polls, media, and text as-is.	send.go
Emit the required meta node for event creation when building event message content.	Detect when the resolved message type is "event" during stanza content construction. Append a child node alongside existing content, mirroring the existing pattern for poll meta nodes. Ensure the new meta node is added before any optional bot node content so ordering matches other meta elements.	send.go

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[sourcery-ai]

Hey - I've left some high level feedback:

• The getTypeFromMessage change always maps EventMessage to type="event"; if there are non-creation event flows (e.g., updates or responses) that reuse EventMessage, consider making the stanza type selection more nuanced or clearly constrained to creation-only usage.
• The meta node injection for type="event" is hard-coded to event_type="creation"; if you anticipate other event lifecycle operations, it may be cleaner to thread the desired event_type from the caller or message contents rather than fixing it here.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The `getTypeFromMessage` change always maps `EventMessage` to `type="event"`; if there are non-creation event flows (e.g., updates or responses) that reuse `EventMessage`, consider making the stanza type selection more nuanced or clearly constrained to creation-only usage.
- The `meta` node injection for `type="event"` is hard-coded to `event_type="creation"`; if you anticipate other event lifecycle operations, it may be cleaner to thread the desired `event_type` from the caller or message contents rather than fixing it here.

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[NeritonDias]

🟢 👍

Contribuição mínima e aditiva: suporte a EventMessage (case event no getTypeFromMessage + nó de meta), 1 arquivo, sem reescrever nada. Obs: este repo não tem branch develop, então avaliei contra a main mesmo. Pode seguir.

@dpaes