fix: use elif in _write_data_file and _read_data_file to avoid redundant checks

[31groot]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:
_write_data_file and _read_data_file in dataset.py used sequential
if statements for mutually exclusive data_format checks. Since
data_format can only match one value at a time, the second if block
was always evaluated unnecessarily. Changed to elif to make the intent
explicit and avoid redundant condition checks.

Which issue(s) this PR fixes:
Fixes #563

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[gemini-code-assist]

Code Review

This pull request updates the dataset helper methods _write_data_file and _read_data_file in core/testenvmanager/dataset/dataset.py to use elif instead of sequential if statements when checking for the CSV data format. The reviewer recommends adding else blocks to both methods to raise a ValueError for unsupported data formats, which prevents silent failures and improves error handling.

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[gemini-code-assist]

While changing the sequential if to elif is a good step, if data_format is an unsupported format (such as JSON or JSONFORLLM), this method will silently do nothing. It is safer to add an else block that raises a ValueError to prevent silent failures and make debugging easier.

Suggested change

	elif data_format == DatasetFormat.CSV.value:
	data.to_csv(data_file, index=None)
	elif data_format == DatasetFormat.CSV.value:
	data.to_csv(data_file, index=None)
	else:
	raise ValueError(f"Unsupported data format for writing: {data_format}")

[gemini-code-assist]

Similarly, if data_format is unsupported, this method will silently return None, which will lead to downstream errors like TypeError when other methods attempt to process the returned data. Adding an else block to raise a ValueError provides defensive programming and clearer error messages.

Suggested change

	elif data_format == DatasetFormat.CSV.value:
	data = pd.read_csv(data_file)
	elif data_format == DatasetFormat.CSV.value:
	data = pd.read_csv(data_file)
	else:
	raise ValueError(f"Unsupported data format for reading: {data_format}")

[31groot]

/assign @jaypume