feat: add call answering, outgoing calls and audio injection

[alpersamur3]

Description

Adds first-class call support: answer incoming calls, place outgoing calls, and
inject audio (e.g. a text-to-speech file) into an active call. Also fixes call
handling that no longer works on current WhatsApp Web.

New Call methods:

• accept() — answer the call (audio only by default, even for incoming video calls)
• end() — hang up an ongoing call
• playAudio(media) — play a MessageMedia/base64 audio clip into the call so the
  other party hears it; resolves when playback finishes, so clips can be chained
• isConnected() — whether the other party has answered

New Client methods:

• call(chatId, { video, waitForAnswer, answerTimeout }) — place an outgoing
  voice/video call; with waitForAnswer: true it resolves once the callee answers
• getActiveCall() — the ongoing call (ringing/placing/connected), or null

Fixes:

• Incoming call event: current WhatsApp Web surfaces calls through
  WAWebCallCollection's activeCall attribute; the previous internal-map set
  hook no longer fires, so client.on('call') never triggered. Now listens to
  change:activeCall (the map hook is kept as a fallback).
• Call.reject(): the previous signaling stanza no longer rejects on current
  WhatsApp Web; it now goes through the voip stack's rejectCall().

Audio is captured from a synthetic Web Audio stream (WhatsApp's getUserMedia is
overridden), so no microphone is required and it works headless. The browser
flags needed for this are added automatically.

Related Issue(s)

fixes #201746

Testing Summary

Test Details

Verified end-to-end against a real phone with live calls.

Outgoing voice call + audio + hang up:

const call = await client.call('1XXXXXXXXXX', { waitForAnswer: true });
if (await call.isConnected()) {
    await call.playAudio(MessageMedia.fromFilePath('./hello.wav'));
    await call.end();
}

Incoming call handling:

client.on('call', async (call) => {
    if (call.fromMe) return;
    await call.accept();                 // or await call.reject()
    await call.playAudio(MessageMedia.fromFilePath('./welcome.wav'));
    await call.end();
});

Confirmed: incoming voice/video calls fire the call event; accept, reject,
end, sequential playAudio and getActiveCall all work; the callee hears the
injected audio; video calls are answered audio-only; everything also works with a
headless browser and no microphone.

Note: the repo's npm test integration suite requires a configured live test
account (WWEBJS_TEST_REMOTE_ID etc.) and only covers messaging/groups, not
calls, so verification was done manually with real calls as shown above.

Environment

• Machine OS: Windows 11
• Phone OS: Android 16
• Library Version: 1.34.7
• WhatsApp Web Version: 2.3000.1017054665
• Browser Type and Version: Google Chrome 149
• Node Version: 22.17.0

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)

Checklist

• My code follows the style guidelines of this project.
• All new and existing tests pass (npm test) — the integration suite needs a live test account and only covers messaging/groups; verified manually with real calls.
• Typings (index.d.ts) have been updated.
• Usage examples (example.js) / documentation have been updated.

[alpersamur3]

Heads-up: I've built video support on top of this, in a follow-up branch (feat/call-video-injection on my fork). It reuses the same synthetic-media approach and adds:

• Call.showImage(media) and Call.playVideo(media, { loop })
• orientation (landscape / portrait / auto) and resolution options on Client.call / Call.accept
• Call.setVideoResolution(px) to change quality mid-call

Verified live end-to-end (portrait/landscape, mid-call source + resolution changes, video + injected audio together), including headless and after a cold restart.

Once this PR is merged I'll rebase that branch onto main and open it as a separate PR to keep the diffs clean — happy to fold it in here instead if you'd prefer.

[alpersamur3]

@aliyss @lindionez Hi! Could you please take a look at this PR when you have a moment? Thanks!