Linked Issue

Closes #5149

Description

The reranker layer currently only supports a fixed allow-list of providers (cohere, sentence_transformer, zero_entropy, llm_reranker, huggingface). There is no way to point Mem0 at a self-hosted or third-party reranker that exposes a Cohere/Jina-compatible /rerank HTTP endpoint (e.g. self-deployed bge-reranker, Jina, Voyage, SiliconFlow, Together, vLLM-hosted rerankers, or internal corporate gateways) — even though the equivalent already works on the LLM/embedding side via openai_base_url.

This PR adds a new openai_compatible reranker provider that speaks the standard rerank HTTP contract:

POST {base_url}/rerank
Authorization: Bearer {api_key}
{ "model": "...", "query": "...", "documents": ["...", "..."], "top_n": N }

Changes

• mem0/configs/rerankers/openai_compatible.py — OpenAICompatibleRerankerConfig extending BaseRerankerConfig with base_url (required), timeout, and headers.
• mem0/reranker/openai_compatible_reranker.py — OpenAICompatibleReranker that POSTs to {base_url}/rerank using httpx, maps each result's index + relevance_score/score back onto the input documents (same return shape as CohereReranker), applies top_k, and falls back to the original order on failure (consistent with existing rerankers).
• mem0/utils/factory.py — register "openai_compatible" in RerankerFactory.provider_to_class.
• Tests — tests/rerankers/test_openai_compatible_reranker.py mocking the HTTP call (config validation, request shaping, score mapping/sorting, top_k truncation, bare-list responses, env-var base_url, error fallback, and factory wiring).
• Docs — new docs/components/rerankers/models/openai_compatible.mdx, an entry in docs/components/rerankers/config.mdx, nav entry in docs/docs.json, and docs/llms.txt index line (verified in sync via scripts/check-llms-txt-coverage.py).

Example usage

config = {
    "reranker": {
        "provider": "openai_compatible",
        "config": {
            "base_url": "https://my-reranker.internal/v1",  # or RERANKER_BASE_URL
            "api_key": "sk-...",                              # or RERANKER_API_KEY
            "model": "bge-reranker-v2-m3",
            "top_k": 5
        }
    }
}

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactor (no functional changes)
• Documentation update

Breaking Changes

N/A — purely additive; a new opt-in provider with no change to existing providers or defaults.

Test Coverage

• I added/updated unit tests
• I added/updated integration tests
• I tested manually (describe below)
• No tests needed (explain why)

All 43 tests in tests/rerankers/ pass (12 new), and ruff check is clean on the changed files.

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have added tests that prove my fix/feature works
• New and existing tests pass locally
• I have updated documentation if needed