check for appReadyMessage before previewing app

[sharon-wang]

• addresses Run App: app may be opened in Viewer before it's ready #5187
• ❓ possibly also addresses Fastapi display in viewer does not work on Ubuntu #4949

Implementation Notes

• adds optional appReadyMessage property to RunAppOptions and DebugAppOptions
• specify an appReadyMessage of 'Application startup complete' for FastAPI and Shiny apps, which leverage uvicorn and output that message when the app is ready
• delete empty DebugConfiguration object from the positron-run-app.d.ts files, as I think they are unused?
• modified the logging level in api.ts for a few messages
• previewUrlInExecutionOutput now checks for both the app url and the appReadyMessage if provided,
  • an app is considered ready if:
    • the appReadyMessage was not provided and the app url has been matched; or
    • the appReadyMessage was provided and matched, and the app url has been matched
  • if the appReadyMessage was provided but not matched, and the app url has been matched, we'll time out of the raceTimeout, but the url will have been matched, so we'll still try to open the app in the Viewer

QA Notes

• I've renabled Python - Verify Basic FastAPI App [C903306] which now passes
• Please use the repro steps from the issue: Run App: app may be opened in Viewer before it's ready #5187

[seeM]

This works well locally in my testing and the approach makes sense to me!