fix: Set files early on mutations to allow clean methods to validate them

[bellini666]

This is a regression from: #394

Fix #564

Summary by Sourcery

This pull request addresses a regression issue by ensuring files are set early in mutations to allow validation methods to function correctly. It also includes a new test to verify the update functionality when unsetting an optional file.

• Bug Fixes:
  • Fixed an issue where files were not set early enough in mutations, causing validation methods to fail.
• Tests:
  • Added a test to verify the update functionality when unsetting an optional file.

[sourcery-ai]

Reviewer's Guide by Sourcery

This pull request addresses a regression issue by setting files early on mutations to allow clean methods to validate them. The changes primarily involve removing the handling of file fields from the prepare_create_update function and its associated calls in the create and update functions. Additionally, a new test case is added to ensure the correct behavior when unsetting an optional file.

File-Level Changes

Files	Changes
strawberry_django/mutations/resolvers.py	Refactored the handling of file fields in the prepare_create_update, create, and update functions to set files early on mutations, allowing clean methods to validate them.
tests/mutations/test_mutations.py	Added a new test case to ensure the correct behavior when unsetting an optional file and fixed a typo in an existing test case.

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[sourcery-ai]

Hey @bellini666 - I've reviewed your changes and they look great!

Here's what I looked at during the review

• 🟢 General issues: all looks good
• 🟢 Security: all looks good
• 🟡 Testing: 1 issue found
• 🟢 Complexity: all looks good
• 🟢 Documentation: all looks good

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.87%. Comparing base (db5ca05) to head (11486f9).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #566      +/-   ##
==========================================
+ Coverage   88.85%   88.87%   +0.02%     
==========================================
  Files          41       41              
  Lines        3544     3533      -11     
==========================================
- Hits         3149     3140       -9     
+ Misses        395      393       -2     

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