Prevent deletion of part which is used in an assembly

[SchrodingersGat]

Adds protection against deleting a part which is a component in an assembly.

@matmair @wolflu05 thoughts here? Should this behaviour be behind a configurable setting?

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[codecov]

Codecov Report

Attention: Patch coverage is 88.46154% with 3 lines in your changes missing coverage. Please review.

Project coverage is 84.00%. Comparing base (2431fc6) to head (a32f04b).
Report is 305 commits behind head on master.

Files	Patch %	Lines
src/backend/InvenTree/InvenTree/models.py	80.00%	1 Missing ⚠️
src/backend/InvenTree/part/models.py	85.71%	1 Missing ⚠️
...venTree/plugin/base/integration/ValidationMixin.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7260      +/-   ##
==========================================
- Coverage   84.06%   84.00%   -0.07%     
==========================================
  Files        1054     1054              
  Lines       46288    46372      +84     
  Branches     1326     1326              
==========================================
+ Hits        38913    38954      +41     
- Misses       7043     7086      +43     
  Partials      332      332              

Flag	Coverage Δ
backend	85.45% <88.46%> (+<0.01%)	⬆️
pui	64.50% <ø> (-0.12%)	⬇️

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[wolflu05]

Sorry that I cannot add some production feedback here, I have not really used assemblies yet. But just from thinking about it, if we delete an part, the part for the assembly is missing, that would mean, we either delete the assembly too, or remove the part from the assembly or deactivate the part somehow that its not shown in search anymore.

[SchrodingersGat]

Sorry that I cannot add some production feedback here, I have not really used assemblies yet. But just from thinking about it, if we delete an part, the part for the assembly is missing, that would mean, we either delete the assembly too, or remove the part from the assembly or deactivate the part somehow that its not shown in search anymore.

I think that this is not quite right. The change I am implementing here prevents the sub_part from a BOM being deleted - which would in practice mean that a BOM item disappears (without warning) from the BOM of an existing assembly. This change prevents the part from being deleted while it is still in the BOM for a different assembly.