DA261 S3 backup end-to-end against MicroCeph

[delgod]

Summary

Adds an end-to-end integration test suite for the S3 backup feature, backed by a MicroCeph-based S3 fixture. Exercises the any-unit guarantee by running create-backup on three different units and list-backups on a fourth, then verifying the resulting objects in the bucket.

Depends on

#59 must be merged first — this PR exercises actions and the relation surface introduced there.

What's in the PR

• tests/integration/backup/__init__.py — package marker
• tests/integration/backup/conftest.py:
  • Module-scoped microceph fixture: snap install microceph, bootstrap, disk add loop,4G,3 --wipe, enable rgw with --ssl-port=445 --ssl-certificate=auto, create a test user, generate a unique bucket name, expose the rgw self-signed cert
  • s3_bucket fixture: boto3 Bucket resource (creates the bucket eagerly, verify=False for the self-signed endpoint)
  • s3_secret_content: access/secret-key dict for the Juju secret used by s3-integrator
• tests/integration/backup/test_s3_backup.py:
  • Deploys 3 valkey units + s3-integrator on Ubuntu 24.04 via jubilant
  • Provisions an s3-creds Juju secret, configures the integrator (bucket, endpoint, region, path, path-style URIs), waits for active/idle
  • Integrates valkey ↔ s3-integrator
  • Runs create-backup on valkey/0 — asserts a backup-id matching %Y-%m-%dT%H:%M:%SZ
  • Runs create-backup on valkey/1 — asserts a distinct id
  • Runs list-backups on valkey/2 — asserts both ids appear in the table in reverse-chronological order
  • Verifies via boto3 that two objects exist under the configured prefix and the first object's body starts with the RDB magic bytes (REDIS or VALKEY)

Notes for reviewers

• The MicroCeph fixture is adapted from canonical/charmed-etcd-operator's tests/integration/backup/conftest.py
  (3.6/edge). The single test deviation from etcd-operator: a --rdb magic-byte verification of the uploaded object replaces etcd's "restore on a new model" round-trip, since restore is not yet implemented.