Add CLI document info preflight

[godlovesmei]

Summary

• Add markitdown --info FILE for lightweight local document metadata as JSON.
• Detect the likely converter using existing StreamInfo guesses and converter accepts() checks.
• Avoid running the full conversion pipeline; unsupported files return JSON with detected_converter: null and a warning.

Fixes #2182

Testing

• uv run --with-editable 'packages/markitdown[all]' python -m pytest packages/markitdown/tests/test_cli_misc.py -q
• uv run --with-editable 'packages/markitdown[all]' python -m pytest packages/markitdown/tests/test_cli_vectors.py -q
• uv run --with-editable 'packages/markitdown[all]' python -m compileall -q packages/markitdown/src packages/markitdown/tests/test_cli_misc.py
• git diff --check

[Copilot]

Pull request overview

This PR adds a lightweight “document preflight” capability to MarkItDown’s CLI and Python API so callers can retrieve basic metadata (and the likely converter) as JSON without running a full conversion.

Changes:

• Add DocumentInfo and MarkItDown.get_local_document_info() to inspect local files and detect an applicable converter via StreamInfo guesses + accepts() checks.
• Add markitdown --info FILE CLI flag to emit the DocumentInfo payload as JSON and exit.
• Add CLI tests covering the JSON shape for a supported XLSX and an unsupported binary file.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
packages/markitdown/tests/test_cli_misc.py	Adds CLI coverage for --info JSON output and unsupported-file warning behavior.
packages/markitdown/src/markitdown/_markitdown.py	Introduces DocumentInfo, preflight inspection API, and refactors converter kwargs construction.
packages/markitdown/src/markitdown/main.py	Implements --info CLI flag and JSON emission path.
packages/markitdown/src/markitdown/init.py	Exports DocumentInfo as part of the public package API.

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[godlovesmei]

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