fix: use newsletter media types for channel media downloads

[themazim]

Description

WhatsApp Web uses separate, unencrypted media types for newsletter/channel content (e.g. newsletter-image instead of image). The downloadMedia() method was passing the regular msg.type to downloadAndMaybeDecrypt, which caused failures for newsletter messages because:

1. Newsletter media is unencrypted, so mediaKey may be null — but the old code required both directPath and mediaKey for direct downloads.
2. Wrong media type: Passing type: "image" instead of type: "newsletter-image" made the download manager select encrypted download entries for unencrypted content, resulting in 404 errors or decryption failures.
3. Internal resolve crashes: The internal msg.downloadMedia() call crashes for newsletter JIDs because getMaybeChat() returns null (newsletter JIDs are not in ChatCollection).

Fix

Uses WA Web's own internal functions to handle newsletter media correctly:

• WAWebMmsMediaTypes.getMsgMediaType(msg) — computes the correct newsletter-prefixed media type (e.g. newsletter-image, newsletter-video, newsletter-gif)
• WAWebMediaCryptoEligibilityUtils.isMediaCryptoExpectedForMediaType(mediaType) — determines whether mediaKey is actually required for the given type

For unencrypted newsletter media, directPath alone is now sufficient to attempt the download. The internal msg.downloadMedia() resolve is skipped for non-chat JIDs (@newsletter, @broadcast) since it cannot resolve these.

Behavior changes

• Newsletter/channel image, video, audio, sticker, ptt, and ptv downloads now work correctly
• Regular (non-newsletter) media downloads are completely unaffected — getMsgMediaType returns the same type as msg.type for regular messages, and isMediaCryptoExpected returns true, preserving the existing directPath + mediaKey requirement
• No changes to the returned MessageMedia object — returns still use type: "image" and mimetype: "image/jpeg", the newsletter-prefixed type is only used internally for the download call

Related Issue(s)

Testing Summary

Test Details

• subscribed to channel and waited for a message with media.
• Verified channel messages can download media as expected
• Confirmed regular chat media downloads remain unaffected

Environment

• Machine OS: Ubuntu 24.04
• Phone OS: Android 14
• Library Version: latest (main)
• WhatsApp Web Version: 2.3000.1036680724
• Browser Type and Version: Chrome (headless)
• Node Version: 22.14.0

Type of Change

• Dependency change (package changes such as removals, upgrades, or additions)
• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Non-code change (documentation, README, etc.)

Checklist

• My code follows the style guidelines of this project.
• All new and existing tests pass (npm test).
• Typings (e.g. index.d.ts) have been updated if necessary.
• Usage examples (e.g. example.js) / documentation have been updated if applicable.