fix: add UTF-8 BOM to CSV transcript exports

[barry47products]

Product Description

CSV transcript exports now open correctly in Excel on both Windows and macOS. Previously, special characters such as curly apostrophes (') and accented letters (é, ü, etc.) were garbled — for example I'm appeared as I’m.

Technical Description

Excel does not respect the Content-Type: text/csv charset header when opening files directly. Without a UTF-8 Byte Order Mark (BOM) at the start of the file, Excel defaults to the system locale encoding (Windows-1252 on most systems), causing multi-byte UTF-8 sequences to be misread.

The fix yields  (UTF-8 BOM) as the first chunk from export_rows_to_csv_stream in apps/experiments/export.py. This is a one-line change to the streaming CSV generator used by StreamingHttpResponse exports.

Migrations

N/A

Demo

Before: downloading a CSV export containing I'm or café and opening in Excel produces I’m / café.

After: characters render correctly.

Docs and Changelog

• This PR requires docs/changelog update

Fixes #697

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 23f7fac2-de00-4f88-8222-b506816573c6

📥 Commits

Reviewing files that changed from the base of the PR and between 1c1d837 and d681a7e.

📒 Files selected for processing (2)

• apps/experiments/export.py
• apps/experiments/tests/test_export.py

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📝 Walkthrough

Walkthrough

This PR adds a UTF-8 BOM (Byte Order Mark) prefix to the CSV export stream in export_rows_to_csv_stream(). The implementation yields a leading "" string before streaming the CSV lines, ensuring Excel and other tools correctly interpret the character encoding. Two new test cases verify that the BOM is emitted as the first chunk and that special Unicode characters are preserved in the output.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: adding UTF-8 BOM to CSV exports to fix character encoding issues in Excel.
Description check	✅ Passed	The description includes all key required sections with comprehensive technical details about the UTF-8 BOM fix, product impact, and issue reference.
Linked Issues check	✅ Passed	The code change directly addresses issue #697 by adding UTF-8 BOM to the CSV export stream, ensuring Excel correctly interprets special characters.
Out of Scope Changes check	✅ Passed	All changes are focused and in-scope: the one-line modification to export_rows_to_csv_stream and corresponding test additions to verify BOM and unicode character handling.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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[snopoke]

Thanks for the contribution @barry47products.

[barry47products]

@snopoke I wanted to check in with you on the CodeScene findings in https://github.com/dimagi/open-chat-studio/pull/3360/checks?check_run_id=76382291625

Is this something you typically address with a refactor or suppress?

[snopoke]

@snopoke I wanted to check in with you on the CodeScene findings in https://github.com/dimagi/open-chat-studio/pull/3360/checks?check_run_id=76382291625

Is this something you typically address with a refactor or suppress?

We are experimenting with CodeScene - you can threat these as quality signal but not compulsory to address, particularly in test code.

[codescene-delta-analysis]

Gates Failed
Enforce advisory code health rules (1 file with Code Duplication)

Gates Passed
3 Quality Gates Passed

See analysis details in CodeScene

Reason for failure

Enforce advisory code health rules	Violations	Code Health Impact
test_export.py	1 advisory rule	10.00 → 9.94	Suppress

Quality Gate Profile: Clean Code Collective
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[codescene-delta-analysis]

❌ New issue: Code Duplication
The module contains 4 functions with similar structure: test_participant_data_export,test_participant_data_export_empty_data,test_participant_data_export_empty_diff,test_session_state_export

_Suppress