fix: panic on Archive event due to unchecked type assertion

[FlavioPulli]

myEventHandler builds postMap without a "data" key, so the *events.Archive case asserting postMap["data"].(map[string]interface{}) (pkg/whatsmeow/service/whatsmeow.go:1820) panics the whole process whenever a chat is archived or unarchived on the paired phone.

Every other case that reads postMap["data"] guards the key first (Message, Receipt, etc.) — Archive was the only one missing the guard. This builds the data payload map directly, preserving the exact webhook shape.

How to reproduce: pair an instance, archive any chat on the phone → the process dies with panic: interface conversion: interface {} is nil, not map[string]interface {}.

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Retargeted to develop (replaces #125), as requested by @iagocotta in #128 (comment).

Note that main and develop have no common ancestor — the GitHub API refuses to compare them (No common ancestor between main and develop) — so the base branch of the original PR could not simply be edited. This branch was created from develop and the commit cherry-picked onto it.

Verified on this branch: go build ./... and go vet ./... both pass.

Summary by Sourcery

Bug Fixes:

• Fix Archive event handling that assumed an existing data payload and crashed on chat archive/unarchive.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Fixes a panic in the Archive event handler by eliminating an unsafe type assertion on the webhook payload and constructing the data map directly.

File-Level Changes

Change	Details	Files
Remove unsafe type assertion on the Archive event webhook payload and build the data map directly to avoid panics when chats are archived or unarchived.	Replace type assertion on postMap["data"] with direct construction of a new map[string]interface{} for Archive events. Populate the Archive event payload map with JID, Timestamp, Action, and FromFullSync fields instead of mutating a preexisting map. Add an explanatory comment describing the previous panic condition and why the payload is now built directly.	pkg/whatsmeow/service/whatsmeow.go

Possibly linked issues

• #: PR changes Archive handler to construct the data map safely, removing the panic described in the issue.
• #Bug: API /chat/archive - interface conversion: interface {} is *events.Archive, not map[string]interface {} (v0.7.2): PR fixes the unchecked type assertion on Archive events that causes the panic reported in the issue.

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