fix: replace pickle with json in RedisCache to prevent deserialization attacks

[spidershield-contrib]

Summary

• Replace pickle.loads()/pickle.dumps() with json.loads()/json.dumps() in RedisCache
• Eliminates arbitrary code execution risk from compromised Redis data

Fixes #169

Problem

pickle.loads() on data from Redis can execute arbitrary code if an attacker gains write access to the Redis instance (CWE-502). This is a well-known deserialization vulnerability.

Solution

Since MCP tool enrichment cache values are JSON-serializable (strings, dicts, lists), json is a safe drop-in replacement that preserves functionality while eliminating the code execution risk.

Changes

File	Change
src/enrichmcp/cache/__init__.py	import pickle → import json, pickle.loads/dumps → json.loads/dumps

Testing

• JSON handles all standard Python types used in MCP tool results (str, dict, list, int, float, bool, None)
• Invalid cached data now raises json.JSONDecodeError instead of potentially executing code