The standalone payload builder imported markdown inside the per-chunk loop
under suppress(ImportError), which read as though the import could fail per
chunk. Try it once before the loop and branch on the result.
_standalone_send seeded `result = None` and returned it after the loop,
suggesting a possible None return. truncate_message always yields at least one
chunk, so the loop always runs; return the failing result directly and the
last result otherwise. No behaviour change.
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