fix: do not create networks and volumes with --dry-run

[Sanjays2402]

Closes #1440

Contributor Checklist:

Podman.run() honors dry_run and only logs the command, but Podman.output() always spawns the process. assert_volume() and assert_cnt_nets() create through output(), so podman-compose --dry-run up really created every referenced network and volume. Both now log the create command and skip it under --dry-run, matching what run() already does for the create/start calls.

Compose spec reference for --dry-run: https://docs.docker.com/reference/cli/docker/compose/#options

Unit tests added in tests/unit/test_dry_run.py: they assert that under dry_run the only podman calls issued are the existence probes (volume inspect / network exists) and never a create. Both fail on current main and pass with this change; the full tests/unit suite is green (469 passed), and ruff check/ruff format are clean on both changed files.

I did not add a newsfragments/ entry because I wasn't sure whether this counts as user-visible enough to warrant one — happy to add newsfragments/dry_run_no_side_effects.bugfix if you'd like it.

This change was prepared with AI assistance; the regression tests were run locally and fail without the fix.

[p12tic]

Looks great, thanks. Please add a release note.

[Sanjays2402]

Added in 53674d0: newsfragments/dry_run_no_network_volume_creation.bugfix, typed as a bugfix since the old behaviour actually created networks and volumes on a dry run.

469 unit tests still pass.