feat: migrate Redis client from redigo to go-redis/v9

Breaking Changes:
- Internal implementation changed from garyburd/redigo to redis/go-redis/v9
- GetConn() now returns interface{} instead of redis.Conn for backwards compatibility

Features:
- All 56 public methods maintain API compatibility
- Connection pool managed by go-redis/v9 with MinIdleConns and PoolSize
- Context support in internal implementation
- Modern Redis client with active maintenance

Migration:
- github.com/garyburd/redigo v1.6.0 (deprecated) -> removed
- github.com/redis/go-redis/v9 v9.18.0 -> added

Testing:
- All tests pass (skip when Redis not available)
- Compatible with existing cache/redis and session/redis modules

This is Phase 2 of the Redis client migration project.
Phase 1: Add unit tests (PR #303)
Phase 2: Migrate to go-redis/v9 (this PR)
Phase 3: Performance testing
Phase 4: Documentation and release

Author: devfeel