fix(core): ensure `terminalOutput` is always a string in task results

[leosvelperez]

Current Behavior

When some tasks fail during execution, it can result in a cryptic:

Failed to convert JavaScript value 'Undefined' into rust type 'String'

This happens because the reported terminalOutput for the errored tasks can be undefined, and it hides the actual error that occurred.

Expected Behavior

Error handling paths should always provide a valid string for terminalOutput, using the error stack/message, or an empty string as a fallback.

Related Issue(s)

Fixes #32675