### Rationale for this change
resolve#39961
### What changes are included in this PR?
When writing a dataset with CsvFileFormat configured with a custom ParseOptions delimiter (e.g. delimiter=">"), the output CSV still used the default "," delimiter. This was because:
- C++ CsvFileFormat::DefaultWriteOptions() always created WriteOptions::Defaults() with delimiter=',', ignoring the parse_options.delimiter stored on the format object.
- Python CsvFileFormat.make_write_options() unconditionally overwrote the C++ write options with a fresh WriteOptions(**kwargs), discarding any C++-side default that might have been set.
Fix the C++ side by propagating parse_options.delimiter into the write options in DefaultWriteOptions(). Fix the Python side by reading the delimiter from the C++ default when the caller does not explicitly specify one, preventing the overwrite from losing the propagated value.
### Are these changes tested?
Yes
### Are there any user-facing changes?
No
* GitHub Issue: #39961
Lead-authored-by: egolearner <lijiliang@outlook.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>
