test: add full-stack integration suite

[alithethird]

What this PR does

• Add monorepo integration suite under tests/integration/
• Add stack-integration tox environment and required integration dependencies
• Include end-to-end whole-mail-system test and full-stack scenarios
• Resolve merge conflicts with origin/main, incorporating improvements from the create-mail-user action: better validation, tenacity-based retries, GDPR helpers, and jubilant upgraded to 1.10.0
• Fix duplicate test path in tox.toml stack-integration environment

Why we need it

Provides a full-stack integration test suite that exercises the complete mail system (Dovecot, Postfix relay, Postfix relay configurator, OpenDKIM) end-to-end in a single tox environment, enabling confidence across the whole monorepo.

Checklist

• I followed the contributing guide
• I added or updated the documentation (if applicable)
• I updated docs/changelog.md with user-relevant changes
• I added a change artifact for user-relevant changes in docs/release-notes/artifacts. If no change artifact is necessary, I tagged the PR with the label no-release-note.
• I used AI to assist with preparing this PR
• I added or updated tests as needed (unit and integration)
• If integration test modules are used: I updated the workflow configuration
  (e.g., in .github/workflows/integration_tests.yaml, ensure the modules list is correct)
• If this is a Grafana dashboard: I added a screenshot of the dashboard
• If this is Terraform: terraform fmt passes and tflint reports no errors
• If this is Rockcraft: I updated the version

Test plan

• All YAML workflow files validated with yaml.safe_load
• All Python files validated with py_compile
• pyproject.toml and tox.toml validated with tomllib
• CodeQL security scan: 0 alerts

Review focus

• tests/integration/conftest.py uses session-scoped Juju fixtures to serve both test_e2e.py and test_configurator_maps.py; private helper functions are replaced with imports from helpers.py
• dovecot-charm/tests/integration/helpers.py now uses tenacity retry decorators and exposes GDPR helpers
• cryptography>=42.0 is kept in integration dependencies for RSA key generation in generate_dkim_keypair()

[alithethird]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Merge conflicts resolved in 538f888. All 14 conflicted files have been resolved, keeping the full-stack integration test additions while incorporating origin/main's improvements (create-mail-user action validation, tenacity retries, GDPR helpers, jubilant 1.10.0 upgrade). Also fixed a duplicate test path in tox.toml (31f4550).