Add integration tests for connection-level behavior

[mack42]

Source: Audit finding (LOW)

Major modules have zero test coverage: `src/server/connection.rs`, `src/server/mod.rs`, `src/command/list.rs`, `src/command/set.rs`, `src/command/hash.rs`, `src/storage/store.rs`. The 79 existing unit tests are concentrated in the parser, RDB, and a few storage internals.

Specifically missing:

• MULTI / EXEC / DISCARD atomicity
• WATCH / EXEC cancellation (per-key version is new in 0.11.0 — needs regression coverage)
• Pub/Sub message ordering and pattern-match dispatch
• Client tracking invalidation (forward + reverse index)
• AUTH / HELLO ACL flows
• Pipelining boundaries

The `redis = "0.27"` dev-dependency is already present. Suggested layout: `tests/integration_*.rs` spawning a server on an ephemeral port and exercising via the real Redis client.

[mack42]

Resolved by #41, which adds the tests/integration_*.rs suite (spawning a real server on an ephemeral port and driving it via the redis client), as suggested here.

Covered: MULTI/EXEC/DISCARD (integration_transactions), Pub/Sub message + pattern-match dispatch (integration_pubsub), AUTH flows (integration_auth), plus strings and collections. The pubsub tests also caught a real message-delivery bug fixed in the same PR.

Not yet covered (can be tracked separately if wanted): client-tracking invalidation and pipelining-boundary cases.