Invalid column reference in Table.AddColumn outputs null instead of an error

Preflight Checklist

• I have installed the latest version of Power Query SDK.
• I have checked existing resources, including the common issues and the release notes.
• I have searched for similar issues.

Power Query SDK

0.2.1

Regression From

No response

Platform

Windows

Architecture

x64

OS Version

No response

VSCode version

No response

PQSdkTool Path

No response

Bug Description

In Power Query, when Table.AddColumn's column generator function references a non-existent column, the output generated for the new column should be an error.

Steps to Reproduce

1. Create a new connector project with a .test.pq file defined as follows (the connector's contents don't matter):

   let
       Source = Table.View(null, [
               GetRows = () => #table({"A"}, {{1}}),
               GetType = () => Value.Type(GetRows())
           ]),
       Added = Table.AddColumn(Source, "B", each if [NonExistent] <> null then 1 else 2)
   in
       Added
   

2. Evaluate the test script file.

Actual Experience

The following two-column table is returned:

A	B
1	null

JSON version of the above:

"Output": [
    {
        "A": 1,
        "B": null
    }
]

Expected Experience

Column B should contain an error, not a null value.

Additional Context

If the same M code is evaluated in Power BI's Query Editor, the following output is produced (reflects the expected error):

Reproduce the issue in a connector project's .test.pq file using the supplied Table.View and Table.AddColumn example; start by tracing how the SDK evaluates the test script and represents cell errors. Done when the missing [NonExistent] reference produces an error in column B rather than null, matching Power BI Query Editor behavior.