test(core): split registry tests into unit and integration scopes

[Tranquility2]

I've wanted to clean this up for a while, core/tests/test_core_registry.py was an outlier.
It used DockerRegistryContainer (a sibling module) to test DockerClient's auth-config wiring, mixing the library under test with its own test setup.
It also carried macOS / SSH-host skip markers because of the insecure HTTP registry it stood up.

This PR splits the coverage along the right seams:

• core/tests/ gets fast, fully-mocked unit tests for the bits testcontainers actually owns: login wiring from docker_auth_config and create()'s pull-on-miss path.
  Runs offline, no skips, no dependency on the registry module.
  Mock setup lives in a small reusable registry_mocks.py helper, constrained with spec=docker.DockerClient so unknown SDK calls fail loudly.
• modules/registry/tests/ gets one real round-trip test that proves auto-login via DOCKER_AUTH_CONFIG actually works, by seeding the private registry and then pulling without explicit credentials.
  Addresses the registry over 127.0.0.1:<port> so the daemon's loopback insecure-registry default kicks in, which means it also works against remote/SSH daemons (no insecure-registries config required).

Drive-by: empty auths ('{\"auths\": {}}') used to raise ValueError and crash DockerClient() init.
It now returns [] ("no credentials configured"), which is what users probably expect.

Hopefully a small step toward making this part of the codebase easier to evolve.

Note for reviewers: please merge rather than squash.
The 3 commits are intentionally standalone (unit-test refactor, auth.py fix + mock tightening, integration test) and are easier to read / bisect kept separate.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.94%. Comparing base (1fadb40) to head (340fef4).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1029      +/-   ##
==========================================
- Coverage   83.06%   82.94%   -0.12%     
==========================================
  Files          16       16              
  Lines        1730     1730              
  Branches      190      190              
==========================================
- Hits         1437     1435       -2     
- Misses        236      239       +3     
+ Partials       57       56       -1     

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