chat.appendStream rejects a request carrying both markdown_text and
chunks with `cannot_provide_both_markdown_text_and_chunks`, so every
native task-card update failed and each turn silently downgraded to the
plain-text fallback message (#87743). The chunks are the native
rendering; the gateway caller keeps its own editable-text fallback rail
for when the native call itself fails, so the markdown_text field was
both rejected and redundant.
Fixes#87743
7fc0c649ff