feat(valkeystore): add Valkey-backed Store

[YimingIsCOLD]

🚀 Summary

Adds session/valkeystore, a Valkey-backed implementation of session.Store for production deployments where session state must outlive a single process. Snapshots are JSON-encoded under a caller-configurable key prefix and expire server-side when their TTL elapses.

✏️ Changes

• New server/internal/session/valkeystore package implementing Prepare, Commit, and Drop against the valkey-glide/go/v2 client; default key prefix session: (override with WithPrefix).
• Tests run against a real Valkey container via testcontainers-go (image valkey/valkey:9.0-alpine, latest version supported by AWS ElastiCache). One container per Test<...> function; per-subtest isolation via t.Name()-derived prefixes.
• TTL-elapsed Prepare test uses polling (50ms TTL, 300ms deadline) instead of an unconditional sleep.
• CONTRIBUTING.md: clarified that want/got may name top-level multi-value captures when no if-init binding is used; semantic names still required when if-init binds want/got.
• memstore_test.go: aligned TestStore_Prepare and TestStore_Commit shape with valkeystore_test.go (folded TestStore_TTL into Prepare, mirrored subtest order, renamed locals to the new want/got pattern).

🧪 Test Plan

• go test ./server/internal/session/... (memstore, valkeystore, and parent session package all pass)
• go test -race ./server/internal/session/memstore/ (concurrent access subtest)
• go test -race ./server/internal/session/valkeystore/ (testcontainer-backed)
• make lint (0 issues)