Config-injected module references for diagnostics owned by other apps.

The doctor framework lives in lemon_core, but several of the things it reports on — media jobs, browser artifacts, language servers — are owned by apps that lemon_core must not depend on. Rather than naming those modules directly, the framework looks them up here and the reference runtime injects them:

config :lemon_core, :doctor_runtime,
  media_jobs: MyApp.MediaJobs,
  lsp_server_manager: MyApp.ServerManager

The umbrella's own registrations live in config/config.exs.

This is the same pattern as :workspace_diagnostics. An unconfigured key returns nil, which callers treat as "that runtime is not available" — the diagnostics degrade to their documented fallbacks instead of failing.