Bug: Extensionless log streams can be rendered as CSV tables

Bug

Extensionless plain-text log streams can be detected as CSV and rendered as a one-column Markdown table.

Reproduction

Given a log-like stream with no filename or extension:

2026-09-12 18:00:01 INFO boot ok
2026-09-12 18:00:02 ERROR disk full

Running through stdin/stream conversion can produce:

| 2026-09-12 18:00:01 INFO boot ok |
| --- |
| 2026-09-12 18:00:02 ERROR disk full |

Expected behavior

When there is no filename, extension, MIME hint, or CSV-style delimiter evidence, decodable plain text should stay plain text instead of being rendered as a CSV table.

Extensionless comma-delimited CSV should still convert to a Markdown table.

Notes

This appears to be a content-detection precedence issue rather than a missing file-extension mapping. Magika can infer text/csv for extensionless text, and CsvConverter currently accepts that before PlainTextConverter can use the detected charset.