feat: add TSV support and delimiter auto-detection to CsvConverter

[anandmt]

Summary

• Extend CsvConverter to accept .tsv files and text/tab-separated-values / text/tsv MIME types
• Auto-detect delimiters using csv.Sniffer with sensible fallbacks (tab for .tsv, comma otherwise)
• Escape pipe characters (|) and newlines in cell values to prevent broken Markdown tables (also addresses [BUG]: CsvConverter produces broken Markdown tables when cell values contain pipe characters (|) #2019)

Motivation

TSV (Tab-Separated Values) files are extremely common in data science, bioinformatics, and LLM pipelines, but are not currently supported by MarkItDown. This is a natural extension of the existing CSV converter.

Changes

• _csv_converter.py: Added .tsv extension and TSV MIME types to accepted formats. Added _detect_delimiter() using csv.Sniffer for auto-detection with fallbacks. Added _escape_cell() to escape pipe characters and newlines in cell values.
• test_files/test.tsv: New test file with tab-delimited data including a pipe character in a cell value.
• _test_vectors.py: New FileTestVector for TSV conversion with must_include checks.

Test plan

• All 312 existing tests pass (0 failures)
• New TSV test vector passes across all test modes (convert_local, convert_stream_with_hints, convert_stream_without_hints, convert_file_uri, convert_data_uri)
• Existing CSV test (test_mskanji.csv) continues to pass — delimiter auto-detected as comma
• Pipe character in TSV cell correctly escaped as \| in output

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[trippinganymess]

I couldn't find text/tsv as IANA recognized MIME type why has this been added? IANA

[trippinganymess]

After doing a little more digging I have found that this implementation is fairly brittle in my opinion.

1. using the csv.sniffer function for checking for delimiter is highly brittle as it deduces the delimiter using a heuristic methods (like frequency counting) it tends to produce false positives and false negative. you don't check if the guess is correct or not. docs
2. If case of csv.Error, you only check if the file's extension is ".tsv" but in the case where the file are coming from Object Storage (AWS S3, Google Cloud Storage), these files usually don't have any extension at all, hence checking the MIME type along with extension is better way to fallback. currently all files without extension during csv.Error simply fallback to CSV type.