fix #19824 where modifications to project models were not saved

[olivierdalang]

Description

Fix #19824 where modifications to project models were not saved.

The fix includes a small refactoring to use a dict instead of a list to store the algorithms in the project provider. This makes it easier to get a reference to the algorithm by name.

Checklist

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[nyalldawson]

Thanks! Can you please add a unit test covering this to python/plugins/processing/tests/ProjectProvider.py ?

[nyalldawson]

Test looks good! It just needs running through the scripts/prepare-commit.sh script in order to correct the formatting