fix(backend): preserve nullable loop item foreign keys

[FicoHu]

Summary by CodeRabbit

• Bug Fixes

  • Improved compatibility with MySQL databases by preserving NULL values in nullable loop-item fields.
  • Ensured automatic status updates and TODO updates only substitute placeholder values where database constraints require them.

• Tests

  • Added coverage confirming nullable fields retain their NULL values during updates.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 6a77eda0-ce3f-40d4-8685-7753305659c3

📥 Commits

Reviewing files that changed from the base of the PR and between 5064867 and e108cb6.

📒 Files selected for processing (3)

• backend/app/models/delivery.py
• backend/app/services/loop_items/service.py
• backend/tests/schemas/test_delivery.py

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📝 Walkthrough

Walkthrough

MySQL loop-item value adaptation now inspects loop_items nullability and caches the result per connection. Inserts, explicit updates, and automatic status transitions apply sentinel values only to non-nullable attributes. Tests cover nullable parent_id and due_at.

Changes

LoopNode null adaptation

Layer / File(s)	Summary
Schema-aware value adaptation backend/app/models/delivery.py, backend/tests/schemas/test_delivery.py	MySQL adaptation accepts a non-nullable attribute set. Schema inspection reads and caches loop_items metadata. Inserts use the detected attributes. Tests preserve None for nullable fields.
Service update integration backend/app/services/loop_items/service.py	Explicit updates and automatic TODO transitions pass the connection-specific non-nullable attributes to dialect adaptation.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Possibly related PRs

• wecode-ai/Wegent#2200: Introduces the MySQL LoopNode null-adaptation logic and update paths extended here.
• wecode-ai/Wegent#2405: Modifies the same delivery adaptation, service, and schema test areas.
• wecode-ai/Wegent#2419: Implements related MySQL nullability introspection and sentinel adaptation changes.

Suggested reviewers: qdaxb

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main change: preserving nullable loop item foreign keys during backend updates.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

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• Create PR with unit tests

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