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.