Add encoder pooling backend foundation

[LxYuan0420]

This PR is:

• To add the encoder pooling runtime path without using decoder KV cache or paged attention.
• To add a minimal native XLM-R/RoBERTa backbone as the foundation for BGE-M3 pooling.
• To keep decoder pooling on the existing generation loader while encoder pooling owns its model-family loader.
• To preserve vLLM’s configured tokenizer and resolved model config contracts.

Note:

This is the PR2 foundation for BGE-M3 support. It does not claim broad XLM-R/RoBERTa model support as a product feature. The local backbone exists because mlx-lm / mlx-vlm do not currently provide a license-compatible encoder implementation for this path.

Follow-up plan:

• PR3: Add BGE-M3 dense embedding behavior on top of this encoder backend.
• PR4: Add BGE-M3 sparse token_classify / lexical-weight output.

[ricky-chaoju]

LGTM, thanks for working on this!