Fix doubled newlines in ANSI-escaped stream output - #14958
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nbformat stores stream `text` as a list of lines that each keep their trailing newline, and fixupStreams concatenates consecutive writes to the same stream. The ANSI-to-HTML branch of mdOutputStream joined those lines with "\n", so every line break was doubled and separate writes (`print(..., end="")` then `print(...)`) got a newline inserted between them. The plain-text path already joins with "", as does JupyterLab. Tracebacks keep their "\n" join: their entries are not consistently newline terminated, and JupyterLab joins them the same way. Fixes #13966
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When rendering Jupyter notebook output containing ANSI escape codes to HTML, doubled newlines appear, showing blank lines between every output line.
Notebook stream output lines already include their trailing newlines. Joining them with an extra newline duplicates the line breaks. The fix joins them with an empty string instead, preserving the original line structure.
Fixes #13966