feat: Add TwelveLabs Pegasus video-to-Markdown converter

[mohit-twelvelabs]

Hi! I'm Mohit, I work at TwelveLabs (@mohit-twelvelabs).

What this adds

An opt-in TwelveLabsConverter that converts video files to Markdown using the TwelveLabs Pegasus video-understanding model.

The built-in AudioConverter only transcribes speech from a video. Pegasus reads the actual frames and audio, so the resulting Markdown can also describe visual scenes, on-screen action, and structure — not just what's spoken. This fits MarkItDown's goal of turning media into LLM-ready text.

• Handles .mp4, .mov, .webm, .mkv, .avi, .m4v.
• New --use-twelvelabs CLI flag (plus --twelvelabs-model / --twelvelabs-prompt), reading the TWELVELABS_API_KEY env var.
• Python API: MarkItDown(twelvelabs_api_key="...", twelvelabs_prompt="...").
• New optional dependency group [twelvelabs]; README documented.

Why it's opt-in / non-breaking

The converter is only registered when an API key is supplied (via the twelvelabs_api_key argument or TWELVELABS_API_KEY), mirroring the existing Document Intelligence / Content Understanding converters. With no key set, nothing changes — all other formats keep using their existing converters, and twelvelabs is an optional dependency guarded by MissingDependencyException.

How it was tested

• New tests/test_twelvelabs_converter.py: mocked unit tests for accepts() routing, convert() request wiring (model/prompt/max_tokens/base64 video), stream-position restoration, missing-dependency and missing-key error paths, and registration on/off based on the key. 25 passed, plus a live API smoke test gated on TWELVELABS_API_KEY (skipped in CI).
• Ran the live path end-to-end against the real Pegasus API with a short clip — the converter base64-uploads the video, Pegasus analyzes it, and returns an accurate Markdown description.
• black (pinned 23.7.0) and mypy clean on the new files.

You can grab a free API key at https://twelvelabs.io — there's a generous free tier.

Note: this is a first-time contribution to this repo, so the Microsoft CLA bot will likely ask me to sign the CLA — happy to do so.

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