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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