Add embedding/response cache with pluggable backends (disk, Redis)

[himanshu231204]

Problem

Once the basic local disk response cache lands (see related issue), larger/team deployments will want a shared cache backend rather than a per-machine disk cache.

Goal

The response cache is generalized behind a small backend interface so it can be pointed at Redis (or similar) without changing pipeline code.

Suggested implementation

Extract a CacheBackend interface from the initial disk-cache implementation (core/cache.py), with a DiskCacheBackend (default) and a RedisCacheBackend (optional extra, following the existing pattern of optional-dependency extras like qdrant/pinecone in pyproject.toml).

Files/components

• openagent_eval/core/cache.py
• openagent_eval/providers/base/* (interface touchpoints)
• pyproject.toml (new redis extra)

Acceptance criteria

• Cache backend is configurable via config.cache_backend, defaulting to disk.
• A Redis-backed cache is demonstrated working in an integration test (using a test Redis instance or fakeredis).

Difficulty

Advanced

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Filed from a repository audit. Full report: https://claude.ai/code/artifact/134cec04-96c5-4d58-af5e-207def4ab524
Depends on the local disk cache issue (#343).