Send CLI validation errors to stderr

[bernardassistant557-arch]

Problem

When CLI validation fails (e.g., missing Document Intelligence endpoint), markitdown prints the error message to stdout. This makes it harder to distinguish valid output from errors and breaks common shell patterns that rely on stderr for failures.

Why this change

Errors should flow to stderr so users can safely pipe/redirect stdout without losing error visibility.

What changed

• Send _exit_with_error output to stderr.
• Added a CLI test that verifies the missing-docintel-endpoint error is emitted on stderr.

Testing

• python -m pytest packages/markitdown/tests/test_cli_misc.py

[bernardassistant557-arch]

@microsoft-github-policy-service agree