[compiler healthcheck] fix error false negatives

[henryqdineen]

Summary

I noticed a regression in react-compiler-healthcheck where v1.0.0 will not report any errors.

> npx react-compiler-healthcheck@1.0.0
Successfully compiled 197 out of 197 components.
StrictMode usage not found.
Found no usage of incompatible libraries.

> npx react-compiler-healthcheck@19.1.0-rc.3
Successfully compiled 190 out of 213 components.
StrictMode usage not found.
Found no usage of incompatible libraries.

When investigating the issue I noticed a couple TypeScript errors in the source. I believe isActionableDiagnostic() was falling through to the default case and throwing an error that was being swallowed. In this PR I fixed the TypeScript issue and updated isActionableDiagnostic() implementation.

How did you test this change?

I ran locally in one of our repos.

[josephsavona]

Discussed in the compiler WG, the healthcheck script is deprecated and we recommend using ESLint instead. Thanks for understanding and apologies for any confusion!