[Feature] Implicitly cast nested `STRUCT` properties during unit test fixture generation

Is this a new bug in dbt-bigquery?

[X] I believe this is a new bug in dbt-bigquery
[X] I have searched the existing issues, and I could not find an existing issue for this bug

Current Behavior

Unit testing models is fine at row level. But when dbt tries to create a fixture for a nested struct, it does not safe cast all the missing properties. Even the nullable properties of any struct need to be inserted manually at the test in order to make it run without errors.

Expected Behavior

When compiling unit tests, dbt's fixture generation should create safe casted nulls recursively for every missing property of nested structs.

Steps To Reproduce

Create this GBQ table:

CREATE TABLE gbq_dataset.example_table(
id INT NOT NULL,
element_name VARCHAR(255),
content ARRAY<STRUCT<name VARCHAR(255), description VARCHAR(255), date TIMESTAMP>>
PRIMARY KEY (id)
);

Create this simple dbt model:
```
{{
config(materialized='table')
}}
```

SELECT * FROM {{ source('gbq_dataset', 'example_table') }}

3. Create this unit_test.yml:
```yml
unit_tests:
  - name: example_model_refresh
    model: example_model
    given:
      - input: source('gbq_dataset', 'example_table')
        rows:
          - {
               id: 1,
               element_name: 'test',
               content: ['struct("John" as name)']
            }
    expect:
      rows:
          - {
               id: 1,
               element_name: 'test',
               content: ['struct("John" as name)']
            }

The resulting compiled SQL will be like this:


/* {"app": "dbt", "dbt_version": "1.8.0b1", "profile_name": "gbq_project", "target_name": "dev", "node_id": "unit_test.gbq_project.example.example_model_refresh"} */

create or replace table `gbq_project`.`gbq_dataset`.`example_model_refresh__dbt_tmp`

OPTIONS(
  expiration_timestamp=TIMESTAMP_ADD(CURRENT_TIMESTAMP(), INTERVAL 12 hour)
)
as (
  select * from (

with dbtcte__example_table as (

-- Fixture for example_table select safe_cast(1 as NUMERIC) as id, safe_cast('test' as STRING) as element_name, (select array_agg(safe_cast(i as struct<name string, description string, date timestamp>)) from unnest([ struct("John" as name) ]) i) as content ) SELECT * FROM dbtcte__example_table ) as __dbt_sbq where false and current_timestamp() = current_timestamp() limit 0

);

5. The error below is shown:
`Invalid cast from STRUCT<name STRING> to STRUCT<name STRING, description STRING, date TIMESTAMP> at [23:37]`

### Relevant log output

_No response_

### Environment

```markdown
- OS: macOS Sonoma 14.4.1
- Python: 3.12.2
- dbt-core: v1.8.0-b1
- dbt-bigquery: v1.8.0-b2

Additional Context

No response

dbt-labs / dbt-bigquery