fix(discord): restore forwarded attachment downloads

[onmax]

Summary

Discord inbound messages created attachment objects without fetchData, so consumers could not download audio or other attachments. The omission affected forwarded Gateway webhooks, REST and history parsing, and direct Discord.js Gateway messages.

Route attachments from all three inbound paths through the adapter's existing rehydrateAttachment() implementation. Existing parser and Gateway tests now assert that the resulting attachments are downloadable.

Test plan

• pnpm validate
• pnpm --filter @chat-adapter/discord test — 278 tests passing
• pnpm --filter @chat-adapter/discord typecheck
• pnpm --filter @chat-adapter/discord build

Checklist

• All commits are signed and verified
• All commits are signed off for the DCO (git commit -s)
• pnpm validate passes
• Changeset added
• Documentation updated (N/A — no public API or configuration change)

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Additional Suggestion:

Direct Gateway (handleGatewayMessage) and REST (parseDiscordMessage) attachment paths build attachments without fetchData, so consumers cannot download attachments received through those paths.