reranking.multimodal (vllm provider only) attaches picture bytes to
synthetic picture chunks before rerank; VLLMReranker sends them as
content-parts documents (base64 data URI + description text) in the
same /v1/rerank request as plain text documents.
Replace the seven repeated `config.reranking.model and ... == provider`
checks and six per-branch ImportError handlers with one None guard and one
try/except around the provider dispatch.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
