Align WhatsApp tests with current channel behavior

[zongtingwei]

Summary

This PR updates src/channels/whatsapp.test.ts to match the current WhatsApp channel implementation.

Specifically, it:

• adds the config mock exports now used by the runtime (ALLOW_WHATSAPP_SELF_MESSAGES, ASSISTANT_NAME, and GROUPS_DIR)
• adds Baileys mocks for media download and version lookup used by the current channel implementation
• updates onChatMetadata assertions to the current three-argument callback shape
• updates the image-message assertion to match the current content-building behavior

Validation

Ran locally:

• npx vitest run src/channels/whatsapp.test.ts
• npx vitest run

Both passed locally.