fix: restore MsgKey._serialized on newer WA Web builds

[netobasilio]

Description

On WA Web 2.3000.1043xxx+ a MsgKey no longer exposes _serialized. The value is
still there, reachable through the minified $1 property and through toString(),
but the named property is gone.

Every page-side read of key._serialized then returns undefined. The most visible one
is the end of WWebJS.sendMessage:

return window.require('WAWebCollections').Msg.get(newMsgKey._serialized);

This becomes Msg.get(undefined), so Client.sendMessage resolves to undefined
even though the message was actually delivered. Callers that check the return value
report a successful send as a failure, and there is no message id to track acks with.
The same pattern breaks editMessage (Msg.get(msg.id._serialized)) and the
lastMessage lookup in getChatModel (chat.lastReceivedKey._serialized).

This PR adds a _serialized getter to WAWebMsgKey.prototype when LoadUtils runs.
The getter returns toString(), so all of these reads work again with a single change
instead of one fallback per call site.

Compatibility with older builds:

• The getter is only installed if the prototype does not already define _serialized.
  Re-injection is a no-op.
• It has a setter that stores an own property. If a build still assigns
  _serialized in the MsgKey constructor, the assignment works as before and the own
  property shadows the getter. A getter-only accessor would make that assignment throw
  in strict-mode code.

Updated after review: the first version only patched the sendMessage call site. The
prototype getter was suggested in the comments and covers every page-side read at once.

Relationship to #201840

#201840 covers the node-side model (getMessageModel, getChatModel,
Message._patch, downloadMedia). This PR does not replace it. A prototype getter is
inherited, not own, so it is not copied when the page builds the plain model it sends
to Node. With this PR alone, sendMessage returns the Message again, but its
node-side id._serialized can still be undefined. normalizeSerialized in #201840
fixes that part. The two changes are complementary and do not overlap.

Related Issue(s)

Related to #201836 (sendMessage returning an empty message id) and #201862.

I have deliberately not used a closing keyword: those reports do not state the WA Web
build precisely enough for me to confirm they are the same root cause.

Testing Summary

Test Details

• Original call-site fix, live session (WA Web 2.3000.1046520132, self-chat, no other
  patches): before, sendMessage returned UNDEFINED; after, sendMessage returned Message. The message was delivered in both runs.
• Prototype getter shape: the same getter (without the setter) has been running in
  production on 2.3000.1046520132 since early September, applied from the application
  side. A reviewer independently reports the same shape fixing sendMessage,
  editMessage and lastReceivedKey on 2.3000.1047868043.
• This exact patch: exercised with stubbed MsgKey classes covering the three
  shapes: new build (no _serialized, value via toString()), old build assigning an
  own _serialized in a strict-mode constructor (value, descriptor and JSON.stringify
  unchanged; re-assignment still works), and a prototype that already defines the
  property (left untouched). LoadUtils was run twice to check idempotency. I have not
  re-run this exact patch against a live session.

npm run check: ESLint clean. Prettier reports only the pre-existing warning on
index.d.ts, which is also present on unmodified main.

npm test could not be run: the suite requires WWEBJS_TEST_REMOTE_ID and a dedicated
paired account.

Environment

• Machine OS: Ubuntu 24.04.4 LTS
• Library Version: 1.34.7 (authored against main @ 942d236)
• WhatsApp Web Version: 2.3000.1046520132
• Browser Type and Version: Chrome/146.0.7680.31 (puppeteer bundled Chromium)
• Node Version: v20.20.2

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). See Test Details: the suite needs a
  dedicated paired test account; npm run check passes.
• Typings (e.g. index.d.ts) have been updated if necessary. No typing change.
• Usage examples (e.g. example.js) / documentation have been updated if applicable.
  Not applicable.

[emrecirik]

Confirmed on WhatsApp Web 2.3000.1047868043 (whatsapp-web.js 1.34.7, puppeteer 24.38.0, Chrome, Node 22).

Dumping the MsgKey of a just-sent message from the page shows the own properties are fromMe, remote, id, self, $1, with $1 = "true_<redacted>@lid_3EB0…_out" and _serialized undefined — so Msg.get(undefined) resolves to undefined while the message is in fact delivered (it shows up in the chat with ack 3).

One suggestion: instead of fixing this single call site, defining a _serialized getter on window.require('WAWebMsgKey').prototype restores every page-side read at once — this same lookup, editMessage (Msg.get(msg.id._serialized)) and chat.lastReceivedKey._serialized all break the same way on this build. I'm running that shape in production and it fixes all three.

[netobasilio]

Switched to the prototype getter in 1a94695, with a setter so builds that still assign _serialized in the constructor keep working. The node-side model still needs #201840.