fix: inject newline between multipart parts missing trailing newline

[jdcc]

When a multipart CSV file part lacks a trailing newline, its last line was being concatenated directly with the first data line of the next part, corrupting row parsing.

• fixes Multipart CSV parsing corrupts rows when a file part lacks a trailing newline #1778

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I don't know if this fix is appropriate for all intended multipart resources, but it works for multipart CSVs.