Omit unset values from Docker SDK filters

[X1pheR]

Summary

Omit unset optional fields when converting typed MCP filter models into Docker SDK filter dictionaries.

Problem

ListImagesFilters(dangling=True) currently becomes {"dangling": true, "label": null}. With Docker SDK 7.1.0 against a live daemon, that changes the result from the expected dangling-image set to an empty result.

Observed against the same daemon:

• {"dangling": true} -> 29 images
• {"dangling": true, "label": null} -> 0 images
• {"dangling": true, "label": []} -> 29 images

The same issue can affect the optional label fields on container and network filters.

Change

Add one small _docker_filters() normalization boundary using model_dump(exclude_none=True) and use it for container, image and network list calls.

The hermetic tests now verify that empty filter objects remain empty and that a dangling-only image filter reaches the Docker SDK as exactly {"dangling": true}.

Validation

• ruff check src tests
• pytest -q: 6 passed
• Independently reproduced against Docker SDK 7.1.0 and a live Docker daemon.