[Feature] Support pseudocolumns in unit tests

### Is this your first time submitting a feature request?

- [x] I have read the [expectations for open source contributors](https://docs.getdbt.com/docs/contributing/oss-expectations)
- [x] I have searched the existing issues, and I could not find an existing issue for this feature
- [x] I am requesting a straightforward extension of existing dbt functionality, rather than a Big Idea better suited to a discussion

### Describe the feature

I would like to be able to define pseudocolumns, like BigQuery's [_file_name](https://docs.cloud.google.com/bigquery/docs/query-cloud-storage-data#query_the_file_name_pseudo-column) pseudocolumn, in unit tests.

### Describe alternatives you've considered

I've considered using the [sql](https://docs.getdbt.com/reference/resource-properties/data-formats#sql) data format for my unit test's input, but I don't want to have to define input values for all columns.

### Who will this benefit?

This feature will be useful for teams that apply logic based on file name in staging models that reference BigQuery external source tables.

### Are you interested in contributing this feature?

With a bit of direction, I'd be happy to write some code!

### Anything else?

Here's a snippet of the unit test I had written:
```yaml
unit_tests:
- name: test_is_valid_file_date_time
description: Confirm that macro correctly parses date and time from file names
model: {model_name}
given:
- input: source('{source_name}', '{table_name}')
rows:
- {_file_name: 2025-12-18.csv, appointment_id: '1'}
- {_file_name: 2025-12-23_05-00-21.csv, appointment_id: '1'}
- {_file_name: 2025-12-18.csv, appointment_id: '2'}
expect:
rows:
- {appointment_id: '1', data_last_updated: '2025-12-23 05:00:21'}
- {appointment_id: '2', data_last_updated: '2025-12-18'}
```

When I ran the test, it failed on this compilation error:
```
14:07:23 Failure in unit_test test_is_valid_file_date_time ({path/to/unit_test}.yml)
14:07:23 Compilation Error in model {model_name} ({path/to/unit_test}.yml)
Invalid column name: '_file_name' in unit test fixture for '{table_name}'.
Accepted columns for '{table_name}' are: [{column_names}]

> in macro format_row (macros/unit_test_sql/get_fixture_sql.sql)
> called by macro get_fixture_sql (macros/unit_test_sql/get_fixture_sql.sql)
> called by model charm_appointments (models/staging/charm/_charm__unit_tests.yml)
```

I believe that [format_row](https://github.com/dbt-labs/dbt-adapters/blob/1e7448f9b6c3440d28fefd5b34b6dcf86c36def1/dbt-adapters/src/dbt/include/global_project/macros/unit_test_sql/get_fixture_sql.sql#L76-L83) raises the error because the `_file_name` column doesn't appear in the information schema for the external source table.