fix: hsn summary calculation

[rtdany10]

Problem:
If you raise a GST invoice with the same item multiple times in the invoice with different UOMs, the report only shows data of 1 uom and skips the rest.

Also, if you add the same item twice in the invoice with different HSN Code(eg: Service items with same name and different HSN code), the report only shows data of either one.

Cause:
HSN wise Summary report grouped data in the SQL query by the following 4 parameters:

1. Parent, ie, Sales Invoice ID
2. HSN Code
3. Item Code
4. UOM

And in the _execute function, we grouped it by:

1. Parent
2. Item Code

And then added only 1 matching data to the report.

This caused issue as only 1 entry from the 1st group was considered by the report.

Solution:
This PR changed the grouping to:
SQL:

1. Parent
2. HSN Code
3. Item Code

_execute function:

1. Parent
2. HSN Code
3. Item Code

Also re-wrote the test case to accommodate this scenario.

[rtdany10]

@sagarvora @vorasmit please review :)

[rtdany10]

@vorasmit 😬

[vorasmit]

Developer Note:

UOM grouping is irrelevant as the report is based on Stock UOM.
One item can only have one stock UOM.