Bugfix: Handle failed type assertions gracefully in completed.go

[jamesgibbons92]

Fixes #6235

getCompletedEvents trusts the outputs in state to be the expected type / shape.
If resource outputs in state contain unexpected fields or types (due to other bugs, or user error), then the cli crashes unexpectedly leading to confusing errors.
These errors are also difficult to recover from without manually altering the state file directly.
This PR just adds purely defenesive code / type assertions to ensure the data is the correct type before acting upon it.
There is no change to functionality or logic.

[jamesgibbons92]

Reproduce the existing bug:

    new sst.Linkable("Linkable", {
      include: ["garbage"], // string is not a valid type in here
    });

Before fix (cli crashes with panic):

After fix (no crash, cli continues to function):