### What problem does this PR solve?
The SiliconFlow `Encode` method sent one HTTP request per text, which is
wasteful and slow when indexing many documents (e.g., 100 docs = 100
round-trips).
SiliconFlow's `/v1/embeddings` is OpenAI-compatible and accepts an array
of strings in `input` (officially documented at
https://docs.siliconflow.cn/en/api-reference/embeddings/create-embeddings,
with a documented max array size of 32). This PR batches the requests up
to that limit, reducing 100 docs to ~4 round-trips, and replaces
`map[string]interface{}` parsing with a typed struct using the same
3-layer validation (count mismatch, out-of-range index, duplicate index)
used in the other drivers.
### Type of change
- [x] Performance Improvement