test(redis): drive backend/redis coverage from 23.5% to 95.1% (#19)

Adds an exhaustive integration + unit test file (coverage_test.go) that
exercises every public surface of the redis provisioning backends:

  - backend.go factories (NewBackend k8s success + fallback, dedicated)
    plus the goredis / k8sEnv / k8sEnvInt helper aliases.
  - local.go: newLocalBackend defaults, Provision against a real Redis
    container (ACL SETUSER + key-prefix isolation + publicHostPort env
    permutations), StorageBytes prefix-sum, Deprovision DELUSER + SCAN,
    ACL-failure fallback path via a closed-port client.
  - dedicated.go: provisionLocal happy + ACL-fallback paths,
    localStorageBytes against real INFO memory, deprovisionLocal
    canonical + legacy username probes, Upstash stubs.
  - k8s.go: every applyXxx CRUD path against a fake.Clientset, all
    Provision rollback branches (network policy, resource quota, secret,
    pvc, service errors) via reactor injection, route-registry write +
    delete error swallowing, Regrade direct-connection happy +
    idempotent + unlimited-tier paths against real Redis, full
    StorageBytes happy path, applyNamespace Terminating-namespace
    recreate + context-cancel branches, helpers (redisK8sRandHex,
    boolPtrR, redisDataVolumeSource), spdyPodExecor.ExecInPod error
    path.

Real Redis dependency is gated by CUSTOMER_REDIS_URL (defaults to
localhost:6379, skips cleanly when no Redis is reachable).

Coverage: 23.5% → 95.1% on instant.dev/provisioner/internal/backend/redis.

Co-authored-by: Manas Srivastava <[email protected]>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mastermanas805