Serialize Redis embedding cache values

[fallintoplace]

Summary

This fixes the Redis-backed embeddings cache so embedding vectors are serialized before being written and deserialized when read back.

Root cause

RedisCacheStore.set() passed Python values directly to redis-py, but embedding cache values are list[float] and redis-py rejects lists with DataError. Even if values were serialized elsewhere, RedisCacheStore.get() returned raw Redis bytes instead of decoded embedding vectors.

What changed

• serialize Redis cache values with JSON on write
• deserialize JSON values on read so embedding cache callers receive normal Python values
• keep a small fallback for legacy non-JSON string values already present in Redis
• add focused tests for Redis cache serialization and embedding round-trips

Validation

• ./.venv/bin/python -m pytest tests/test_cache_embeddings.py