Discord 纯媒体发送去掉空 content 避免 400（PR #9507）

• You use Discord media sending and have seen occasional 400 errors on media-only payloads.
• Runtime can be upgraded to include PR #9507.

1. Upgrade to a build including PR #9507 and restart gateway.
2. Run three cases: media-only, media+caption, and text-only, then capture HTTP status for each.
3. Inspect outbound payloads to confirm media-only request omits empty `content` field.
4. Keep one regression test for reply/reference path to ensure message_reference still works when provided.
5. Monitor error-rate trend for 24h to validate fix impact in production traffic.

• Large files and CDN/transcoding limits can still cause failures unrelated to empty content fields.
• Proxy/middleware rewriting multipart payloads may reintroduce similar symptoms（需验证）。

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