test: integration tests (TCP client against running server)

[zulerne]

Add end-to-end integration tests that start the server and communicate via TCP using the RESP protocol.

Approach

• Start server on a random port in TestMain or per-test
• Use net.Dial to connect as a Redis client
• Send RESP-encoded commands, assert RESP-encoded responses
• Test command sequences (SET then GET, RPUSH then LRANGE, etc.)
• Test concurrent client connections
• Test graceful shutdown behavior

Files

• `internal/server/server_test.go` — integration tests
• Helper: RESP client for tests (or use `pkg/resp` directly)