fix: buffer CLI stdin before format detection on Windows

[Mo Hui (mohui666)]

Fixes #1209

What changed

• buffer CLI stdin into BytesIO before conversion so format probing can rewind it on Windows
• make the existing stdin vector test actually exercise stdin
• add a regression for a pipe that reports seekable but cannot rewind
• run CLI subprocess tests with the active interpreter and read UTF-8 output as UTF-8

Root cause

On Windows, pipe-backed sys.stdin.buffer can report seekable() == True even though seek(0) does not rewind it. Magika consumes the pipe during format detection, leaving the converter at EOF. This is why the CLI returned an empty result even for ASCII HTML.

Verification

• real PowerShell pipe: HTML piped to python -m markitdown -x html produces # Test HTML
• direct raw-byte subprocess stdin: return code 0, expected stdout, empty stderr
• targeted stdin regressions: 13 passed
• CLI tests: 41 passed, 12 skipped
• package tests: 307 passed, 33 skipped, with the unchanged Windows-only URI path assertion deselected
• Black, compileall, and git diff --check pass

The only native-Windows full-suite failure is the pre-existing test_file_uris assertion, which expects the Unix path /path/to/file.txt instead of Windows C:\path\to\file.txt.

[Mo Hui (mohui666)]

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[Mo Hui (mohui666)]

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[Copilot]

🟢 Approval recommended

The Windows stdin failure is addressed with appropriate regression coverage; only a minor duplicate import remains.

Pull request overview

Buffers CLI stdin before conversion to ensure reliable format detection and rewinding on Windows.

Changes:

• Buffers stdin using BytesIO.
• Corrects stdin test execution and adds a Windows pipe regression test.
• Uses the active interpreter and explicit UTF-8 file decoding.

File summaries

File	Description
packages/markitdown/src/markitdown/__main__.py	Buffers stdin before conversion.
packages/markitdown/tests/test_cli_misc.py	Adds Windows pipe regression coverage.
packages/markitdown/tests/test_cli_vectors.py	Corrects stdin and subprocess test behavior.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 1
• Review effort level: Balanced

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[Copilot]

🟢 Approval recommended

The implementation addresses the Windows rewind failure and includes appropriate regression coverage.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 0 new
• Review effort level: Balanced