docs: EncHandler wasm portability (whatsapp-rust#793)

[jlucaso1]

Summary

Documents the fix from oxidezap/whatsapp-rust#793, which relaxed EncHandler's Send + Sync supertrait to MaybeSendSync and added a wasm32-gated async_trait(?Send).

Files changed:

• changelog/2026-06-09.mdx — Added an EncHandler wasm portability entry to the "Fixes & hardening" section.
• api/bot.mdx — Added a <Note> to the with_enc_handler section explaining that !Send JS handles are allowed on wasm32, while native targets keep Send + Sync via MaybeSendSync.

What changed in the library

EncHandler (the custom enc extension point registered via BotBuilder::with_enc_handler) previously required Send + Sync unconditionally, making it impossible to implement with !Send JS handles on the wasm32 port. The fix mirrors how EventHandler and SendContextResolver already handle this: use MaybeSendSync as the supertrait and gate async_trait with ?Send on wasm32. Native behaviour is unchanged.

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📒 Files selected for processing (2)

• api/bot.mdx
• changelog/2026-06-09.mdx

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