Fix #2390: don't silently truncate CSV columns, handle preamble row

[Mohamed Elfeky (FekyBaz)]

Fixes #2390.CsvConverter treated rows[0] as header unconditionally and silently truncated wider data rows via row[:len(rows[0])].Adds two behaviors:- Preamble heuristic: lone single-field first row followed by a wider row is emitted as plain text and the second row becomes the header (covers WizTree exports whose line 1 is a banner and line 2 is the real header).- No silent loss otherwise: table widens to max row width with a UserWarning instead of truncating.Also avoids mutating parsed rows in place.Short rows are still padded; empty inner rows keep existing behavior.Verified with WizTree repro plus existing CSV expectations (BOM, blank rows, pipe escaping).

[Mohamed Elfeky (FekyBaz)]

@microsoft-github-policy-service agree

[afourney]

I agree that we should expand the columns to match the widest row. However, I don't want to overfit to WizTree. I will address the more general case, and recommend that people implement a custom plugin for WizTree specifically.

[afourney]

Addressed on #2430

[Mohamed Elfeky (FekyBaz)]

Thanks afourney — the general fix in #2430 covers the core issue (no more silent truncation), and I agree the WizTree preamble is better left to a custom plugin rather than overfitting the converter. Closing this in favor of the merged approach makes sense.