Precreate egg-info directory for runtime image

[Teagan42]

What

• add a Dockerfile regression test that ensures the runtime stage precreates the mcp_plex.egg-info directory before ownership changes
• update the Dockerfile to create /app/mcp_plex.egg-info during the build and bump version metadata to 2.0.42 across manifests and the lockfile
• refactor the Dockerfile artifact tests with a directive lookup helper for clearer assertions

Why

• the runtime container still attempted an editable install at startup and failed to create mcp_plex.egg-info, so the build path must exist with the correct ownership ahead of time

Affects

• Dockerfile runtime stage
• Docker artifact tests and project metadata files

Testing

• uv run ruff check .
• uv run pytest

Documentation

• n/a

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https://chatgpt.com/codex/tasks/task_e_68fa09919df48328a5db09588741116b