fix(plugin): use app_name instead of plugin_path when deregistering models

[nino-tan-smartee]

Summary

• _deactivate_mixin uses plugin_path (full dotted module path) as the key into apps.all_models when removing plugin models during reload
• Django registers models under the app_label (short app_name), not the full plugin_path
• For plugins with nested module paths (e.g. myplugin.myplugin), plugin_path != app_name
• Since apps.all_models is a defaultdict, looking up plugin_path silently creates an empty OrderedDict, then .pop(model) raises KeyError
• This causes recurring KeyError crashes every plugin reload cycle (~1 minute) for any external plugin with a nested package structure

Fix

• Use app_name (already computed at line 98) instead of plugin_path on line 118
• Add default None to .pop() for defensive safety
• Consistent with line 123 which already correctly uses app_name

Reproduction

1. Create an external plugin with a nested module structure (e.g. purchase_quotation/purchase_quotation/)
2. Install and activate the plugin
3. Observe recurring KeyError: '<model_name>' in error reports every plugin reload cycle

Test plan

• Verify external plugins with nested module paths no longer crash during reload
• Verify built-in plugins continue to work correctly (no regression)
• Verify plugin model cleanup still works (models are properly deregistered)

from setuptools import find_packages, setup

setup(
    name="inventree-purchase-quotation",
    version="0.1.0",
    description="RFQ and Purchase Quotation management for InvenTree",
    author="....",
    packages=find_packages(),
    entry_points={
        "inventree_plugins": [
            "PurchaseQuotationPlugin = "
            "purchase_quotation.plugin:PurchaseQuotationPlugin",
        ],
    },
    install_requires=[
        ...
    ],
)

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[matmair]

@nino-tan-smartee thank you for the PR; please add a unit test that ensures the fixed issues does not appear again

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.08%. Comparing base (97aec82) to head (3a76fac).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #11536      +/-   ##
==========================================
- Coverage   88.08%   88.08%   -0.01%     
==========================================
  Files        1296     1296              
  Lines       59182    59182              
  Branches     1934     1934              
==========================================
- Hits        52133    52132       -1     
- Misses       6569     6570       +1     
  Partials      480      480              

Flag	Coverage Δ
backend	89.26% <100.00%> (-0.01%)	⬇️

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Components	Coverage Δ
Backend Apps	91.70% <100.00%> (ø)
Backend General	93.43% <ø> (ø)
Frontend	70.92% <ø> (ø)

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[nino-tan-smartee]

@nino-tan-smartee thank you for the PR; please add a unit test that ensures the fixed issues does not appear again

@matmair I added the unit test