Invalid Source Line in diff error message when using variable with newline

The following textual response

line1
line2
line3
line4

can be tested with the multiline body in Hurl.:

1     GET http://localhost:8000/test
2     HTTP 200
3    ```
4    line1
5    line2
6    line3
7    line4
8    ```

If LINE4 is returned by the server (instead of line4), we have the following error message (as expected):

    --> test.hurl:5:1
    |
    | GET http://localhost:8000/test
    | ...
  7 | line4
    |   -line4
    |   +LINE4
    |

The invalid source line can occur when the expected body contains variables with newline value.
For example:

1  GET http://localhost:8000/test
2  [Options]
3  variable: newline=\n
4  HTTP 200
5  ```
6  line1
7  line2{{newline}}line3
8  line4
9  ```

The error has now an invalid source line:

  --> /tmp/test.hurl:9:1
  |
  | GET http://localhost:8000/test
  | ...
9 | ```
  |   -line4
  |   +LINE4
  |

```

Reproduce the invalid source-line output with the multiline Hurl example and a newline-valued variable, then trace the diff error formatting path. Done means the reported source line points to the actual mismatching body line rather than the closing code fence, with regression coverage for this case.