Add TwelveLabs Marengo embedding provider

[mohit-twelvelabs]

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

What this adds

A new opt-in embedding provider, TwelveLabsEmbeddingProvider, backed by TwelveLabs Marengo. Marengo produces 512-dimensional embeddings in a shared multimodal space, so text queries can retrieve over video/image/audio content embedded with the same model. This PR implements the text path used by R2R's search and ingestion pipelines (sync + async), mirroring the existing Ollama/OpenAI providers.

Why it helps R2R

R2R already supports several text embedding backends. Marengo extends R2R toward multimodal RAG: the same provider can embed text queries into the same vector space TwelveLabs uses for video/audio, which is a natural fit for indexing rich media alongside documents.

How to use it (opt-in, non-breaking)

[embedding]
provider = "twelvelabs"
base_model = "marengo3.0"
base_dimension = 512

Set TWELVELABS_API_KEY in the environment. Defaults and all existing providers are unchanged; nothing is wired in unless you select provider = "twelvelabs". The twelvelabs SDK import is lazy, so it's only required when the provider is actually instantiated.

Changes

• core/providers/embeddings/twelvelabs.py — new provider (sync + async)
• Registered in the embedding factory, supported_providers, and provider __init__ exports
• twelvelabs >=1.2.8 added to the core optional dependencies
• Config reference example in all_possible_config.toml
• tests/unit/test_twelvelabs_embedding.py — config/validation unit tests (no network) plus an API-key-gated live test asserting a 512-dim embedding

Testing

• Ran the focused suite in a Python 3.12 env with the full [core] extras installed: all 4 tests pass, including the live Marengo call returning a 512-dim vector through the provider class.
• ruff format and ruff check pass on all changed files.

Per CONTRIBUTING, this is opened directly for review; happy to also file an issue or iterate on anything. You can grab a free API key at https://twelvelabs.io — there's a generous free tier.