fix: race in TestProcessCommandAgentConnected + 6 lint issues

[JoshuaAFerguson]

Summary

Two pre-existing CI failures newly visible after PR #255 / #256 made the lint and test infrastructure functional:

1. Second sqlmock race

`TestProcessCommandAgentConnected` raced on go-sqlmock for the same reason `TestUpdateAgentHeartbeat` did (PR #256) — `go hub.Run()` goroutine inside `sqlmock.exec` while the test goroutine calls `mock.ExpectExec`. Apply the same `ExpectationsWereMet` polling pattern.

2. golangci-lint v2.5 findings (newly running)

File	Issue	Fix
`agent_vnc_tunnel.go:179`	unchecked `tunnel.conn.Close()`	`_ = tunnel.conn.Close()`
`main.go:238`	unchecked `a.wsConn.Close()`	`_ = a.wsConn.Close()`
`main.go:536`	unchecked deferred `resp.Body.Close()`	`defer func() { _ = ... }()`
`main.go:583,587`	unchecked `retryResp.Body.Close()` (×2)	`_ = retryResp.Body.Close()`
`internal/errors/errors.go:28`	ST1005 capitalized "Kubernetes resource not found"	lowercase to "kubernetes…"

Verified

• `go test -race -run TestProcessCommandAgentConnected -count=10 ./internal/services/` clean (10/10)
• `go build ./...` clean in both modules

Deferred

Other test files (`agent_hub_redis_test.go`, `websocket_enterprise_test.go`, …) use the same `time.Sleep` + `mock.ExpectExec` pattern. They have not yet failed under `-race` in CI; defer until they do (or a sweep PR if there's appetite).