Fix build item over-allocation checks

[fuzeman]

This pull request fixes an issue with BuildItem over-allocation checks which prevent allocation changes to over-allocated items. Looking a bit closer at related code it appears SalesOrderAllocation already handled this correctly, and I've aligned these changes with that implementation.

---

Not sure if this is a concern but I noticed the quantity checks between BuildItem and SalesOrderAllocation don't appear to be aligned with one casting to decimal.Decimal and the other not. Not sure if this is breaking anything, but it doesn't look quite right to me.

https://github.com/inventree/InvenTree/blob/f8c7635a8d42f8860dc39198d7962f5286bcd853/src/backend/InvenTree/build/models.py#L1609-L1617 https://github.com/inventree/InvenTree/blob/f8c7635a8d42f8860dc39198d7962f5286bcd853/src/backend/InvenTree/order/models.py#L2014-L2025

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Codecov Report

Attention: Patch coverage is 96.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 84.25%. Comparing base (f8c7635) to head (c3b6369). Report is 312 commits behind head on master.

Files with missing lines	Patch %	Lines
src/backend/InvenTree/stock/models.py	85.71%	1 Missing :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8235   +/-   ##
=======================================
  Coverage   84.24%   84.25%           
=======================================
  Files        1157     1157           
  Lines       52347    52373   +26     
  Branches     1892     1892           
=======================================
+ Hits        44100    44125   +25     
- Misses       7789     7790    +1     
  Partials      458      458           

Flag	Coverage Δ
backend	86.01% <96.66%> (+<0.01%)	:arrow_up:

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[SchrodingersGat]

@fuzeman thanks for the fix, looks like a pretty subtle one to work out!

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Status	Branch	Result
✅	0.16.x

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