Support Voyage contextual embeddings

[williambarberjr]

Changes

• add voyage-context-3 to the VoyageAI model catalog
• add grouped contextualized_embed / acontextualized_embed APIs that preserve document groups
• keep flat embed(model="voyage-context-3", ...) working by routing it through the contextual endpoint with one group per input

Validation

• Rust test suite passes
• Python contextual binding smoke tests pass
• live Voyage smoke: grouped inputs returned [2, 1] embeddings at 256 dims, and flat embed(...) returned 2 embeddings at 256 dims

Notes

• the full existing Python binding test file still has unrelated proxy/CA expectation failures on current main

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