Fix unnecessary quotes showing up in tsv/csv files

[man-shu]

To reproduce, try:

import ibc_api.utils as ibc
db = ibc.get_info("raw")
participants = db[db["path"] == "participants.tsv"]
downloaded_db = ibc.download_data(filtered_db)

The downloaded participants.tsv would have unnecessary "" around each row:

"participant_id	age	sex	handedness score"

This fails BIDS validation.

[Copilot]

Pull request overview

This PR fixes incorrect CSV/TSV handling during downloads that caused TSV output to be re-serialized with unnecessary quoting (e.g., the entire header/row wrapped in ""), which breaks BIDS validation.

Changes:

• Determine delimiter (sep) based on the source file extension (.csv vs .tsv) when reading downloaded tabular data.
• Reuse the same delimiter when writing the local output via DataFrame.to_csv(...), eliminating the accidental “single-column TSV” re-serialization that triggered quoting.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.