fix: OneToManyInput saves and runs validation on foreign key #487

[keithhackbarth]

Description

Fix for issue described in #487

Notes:

• Used django_assert_max_num_queries to write test
• Reduced the number of queries for a "patch" update from 25 to 12
• test_relation.py was blank so deleted it.
• As a new contributor, I found it confusing which tests go where so added docstring to test_relationship to clarify my understanding

Types of Changes

• [ ] Core
• [x] Bugfix
• [ ] New feature
• [ ] Enhancement/optimization
• [ ] Documentation

Issues Fixed or Closed by This PR

• https://github.com/strawberry-graphql/strawberry-django/issues/487

Checklist

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 87.92%. Comparing base (bd75a3b) to head (274c45c). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #490 +/- ## ========================================== + Coverage 87.91% 87.92% +0.01% ========================================== Files 37 37 Lines 3170 3173 +3 ========================================== + Hits 2787 2790 +3 Misses 383 383 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[keithhackbarth]

@bellini666 - Good call. Updated.