Improve reporting of "fn is not a function" error

[josephjclark]

Short Description

This PR reports a better error for the extremely frustrating "fn is not a function" error that creeps up when running a job like:

console.log('hi')

Fixes #811

Implementation Details

The problem occurs when the operations array that gets passed to the core execute reducer includes a non-function value.

When doing console.log() at the top level, we compile like this:

export default [console.log()]

Obviously console.log returns undefined, so this may as well be:

export default [undefined]

Which is an error in the runtime.

The validation catches this case and (hopefully!) returns a clearer error message - although of course this is a hard thing to explain, and it's hard to generate an error position at this point, so I have to be a bit vague.

QA Notes

Output in CLI:

[CLI] ♦ Versions:
         ▸ node.js                    22.12.0
         ▸ cli                        1.30.3
         ▸ @openfn/language-common    latest
[CLI] ♦ Imported 8 env vars from .env file
[CLI] ✔ Installing packages...
[CLI] ✔ Installation complete in 779ms
[CLI] ✔ Compiled all expressions in workflow
[JOB] ℹ hello
[R/T] ✘ job-1 aborted with error (1ms)

[R/T] ✘ CRITICAL ERROR! Aborting execution
[R/T] ✘ InvalidOperationError: Invalid operation at statement 0
[R/T] ✘ Ensure all top-level function or method calls are wrapped in an operation, like fn()

[CLI] ♦ Workflow failed in 837ms.
 ELIFECYCLE  Command failed with exit code 1.

Output in Worker:

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