feat: session Redis store adapter

[FumingPower3925]

Summary

Add a Redis-backed session.Store implementation that uses celeris's native Redis driver.

Context

The session middleware already defines a Store interface:

type Store interface {
    Get(ctx context.Context, id string) (map[string]any, error)
    Save(ctx context.Context, id string, data map[string]any, expiry time.Duration) error
    Delete(ctx context.Context, id string) error
    Reset(ctx context.Context) error
}

Currently only MemoryStore is provided. For multi-instance deployments, sessions must be shared via an external store.

Scope

• Implement RedisStore satisfying session.Store
• Use celeris's native Redis driver (v1.4.0 feat: RESP2/RESP3 parser (zero-alloc) #132-test: Redis driver conformance + benchmark vs go-redis #137) — not go-redis
• Serialize session data as JSON (or msgpack with option)
• Support TTL via Redis SETEX/PSETEX
• Reset() uses SCAN+DEL with key prefix (not FLUSHDB)
• Key prefix configurable (default: sess:)

Design Principle

The store adapter should depend on a driver-agnostic Store interface so that future drivers (Memcached, DynamoDB, etc.) can implement the same interface without changing the session middleware.