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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