fix(cli): allow Content Understanding conversion from stdin

[Tyyyy (uczltw6)]

Summary

• preserve the CLI's filename-optional stdin mode when --use-cu is enabled
• let piped input use the existing convert_stream() dispatch path
• add a regression test for Content Understanding stdin routing

Problem

The CLI documents piping input without a filename, but the --use-cu branch exits before reaching the shared stdin path. Content Understanding analyzes binary input and does not require a local file path, so this guard unnecessarily prevents cat report.pdf | markitdown --use-cu --cu-endpoint ....

The endpoint validation remains unchanged, and filename-based conversion is unaffected.

Tests

• hatch test tests/test_cu_converter.py tests/test_cli_misc.py -q — 127 passed
• hatch test -q — 337 passed, 4 skipped
• Black 23.7.0 on both changed files — passed

OpenAI Codex assisted with identifying the inconsistent CLI branch and preparing the focused regression test; the behavior and test results were verified locally.

[Copilot]

Pull request overview

Enables Content Understanding conversion for piped stdin input.

Changes:

• Removes the filename requirement for --use-cu.
• Adds regression coverage for stdin routing through convert_stream().

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/markitdown/src/markitdown/__main__.py	Allows --use-cu without a filename.
packages/markitdown/tests/test_cu_converter.py	Tests Content Understanding stdin dispatch.

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[afourney]

Merged. Thanks.