[Bug] Unit tests: macro fails when 'this' is passed as target_relation ('str' object has no attribute 'database')

### Is this a new bug in dbt-core?

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

### Current Behavior

When running dbt unit tests, a macro that works correctly during dbt run fails during unit test execution.

The failure happens when the macro receives `this` as an argument. In unit test context, this does not behave like a proper Relation object and does not expose attributes such as database, schema, or identifier.

As a result, macros that rely on Relation attributes fail with the error:

`'str' object has no attribute 'database'`

### Expected Behavior

Unit tests should behave consistently with runtime execution (dbt run).

Specifically, one of the following should be true:

this resolves to a proper Relation object in unit tests, or

there is a documented and supported way to safely access Relation metadata in unit test context, or

macros can reliably detect unit test context and avoid breaking when accessing Relation attributes.

### Steps To Reproduce

1.Create a model that calls a macro and passes this as an argument:

```
{{ detect_changes(
target_relation = this,
source_table = ref('test_macro_detect_changes_input'),
compare_columns = ['external_id', 'updated_at', 'username', 'email', 'user_status']
) }}
```

2. Inside the macro, access Relation attributes, for example:

`target_relation.database`

3. Define a unit test for the model and use this as one of the test inputs:

`- input: this`

4. Run the unit test:

`dbt test --select test_first_run_detect_changes`

5. Observe the compilation error:

`'str' object has no attribute 'database'`

### Relevant log output

```shell
07:05:06 Failure in unit_test test_first_run_detect_changes_braze (tests\unit\fixtures\test_detect_changes.yml)
07:05:06 Compilation Error in unit_test test_first_run_detect_changes (tests\unit\fixtures\test_detect_changes.yml)
'str object' has no attribute 'database'

> in macro detect_changes (macros\detect_changes.sql)
> called by unit_test test_first_run_detect_changes (tests\unit\fixtures\test_detect_changes.yml)
```

### Environment

```markdown
- OS: Windows 11
- Python: 3.12.0
- dbt: 1.10.11
```

### Which database adapter are you using with dbt?

bigquery

### Additional Context

_No response_