agent: return port attributes from devcontainer

We're currently missing `portAttributes` in the devcontainers response:

https://containers.dev/implementors/json_reference/#port-attributes

Fetch this from the devcontainer metadata Docker container label and return it in the response.

Start by tracing the devcontainer response and the code that reads labels from the metadata Docker container. Read the containers.dev port-attributes reference to confirm the response shape, then verify that the returned response includes the metadata label value and add or run the relevant response tests if they exist.

Vom Indexierungsmodell aus dem Issue-Text verfasst.