fix: insert child with preset pk in reverse relation add()

[moneebullah25]

Fixes #1819

Problem

owner.children.add(child) on a reverse FK called child.upsert(). When the child already had a pk set (a UUID default, or an explicit id with autoincrement=False), upsert() took the update path and no row was inserted, so the add silently did nothing.

Change

RelationProxy.add() now calls item.upsert(__force_save__=True), the same flag already used in save_mixin.py and model.py. A child with a preset pk is inserted, and a child that already exists in the database is still updated.

Tests

Added to tests/test_queries/test_adding_related.py:

• child with a default UUID pk is inserted
• child with an explicit non-autoincrement pk is inserted
• an existing (already saved) child is updated
• a fresh instance with an existing pk updates that row

What was run

• ruff format --check and ruff check on both changed files: pass
• mypy ormar tests/test_queries/test_adding_related.py --ignore-missing-imports: pass
• pytest tests/test_queries/test_adding_related.py (sqlite): 5 passed
• Full suite and the postgres/mysql backends were not run.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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[codspeed]

Merging this PR will not alter performance

✅ 114 untouched benchmarks
🆕 9 new benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
🆕	test_adding_new_children_to_reverse_relation[10]	N/A	84.5 ms	N/A
🆕	test_adding_new_children_to_reverse_relation[20]	N/A	168.4 ms	N/A
🆕	test_adding_new_children_to_reverse_relation[40]	N/A	337.3 ms	N/A
🆕	test_adding_saved_children_to_reverse_relation[10]	N/A	110.1 ms	N/A
🆕	test_adding_saved_children_to_reverse_relation[20]	N/A	221.1 ms	N/A
🆕	test_adding_saved_children_to_reverse_relation[40]	N/A	440.4 ms	N/A
🆕	test_saving_related_new_children[10]	N/A	91.4 ms	N/A
🆕	test_saving_related_new_children[20]	N/A	174.7 ms	N/A
🆕	test_saving_related_new_children[40]	N/A	340.9 ms	N/A

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_{Comparing moneebullah25:fix/relation-add-insert-child-with-pk (a9a2967) with master (310df0c)¹}

Footnotes

1. No successful run was found on master (ee66c82) during the generation of this report, so 310df0c was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[collerek]

Thanks :)