[WIP] Add dedupe-before-write behavior for collection and junction-table paths

[Codex]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

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This section details on the original issue you should resolve

<issue_title>Prevent duplicate inserts for set-like child/link rows</issue_title>
<issue_description>## Summary
Add dedupe-before-write behavior for collection and junction-table paths where duplicates are semantically equivalent.

Scope

• Deduplicate repeated items within a single request
• Prevent re-adding rows already linked to the parent aggregate
• Apply changes in service/repository layers
• Preserve existing API responses

Deliverables

• Updated write logic for set-like/link-table paths
• Unit and integration coverage for silent duplicate handling

Acceptance criteria

• Duplicate-equivalent requests do not attempt duplicate inserts
• Existing success responses remain unchanged
• No duplicate rows are created for covered paths

Testing

• Unit tests for request-level dedupe
• Unit tests for update flows that previously re-added existing links
• Integration tests proving duplicate-equivalent requests remain successful

Dependencies

• Depends on the duplicate-handling policy sub-issue

Parent: #887</issue_description>

Comments on the Issue (you are @codex[agent] in this section)

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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