check for appReadyMessage before previewing app (#5265)

[sharon-wang]

• Addresses #5281 to bring the fix for https://github.com/posit-dev/positron/issues/5187 to the patch branch
• Backports the commit from #5265 to the prerelease/2024.11 branch for the 2024.11.1 pre-release patch

[sharon-wang]

Merging since this was already approved in https://github.com/posit-dev/positron/pull/5265 and verified on main in https://github.com/posit-dev/positron/issues/5187.