Test/PROD-1439 duplicate content mapping guards

[5PK]

Summary

Adds unit tests for the two duplicate-mapping guards introduced in PR #143 (Jira ticket PROD-1439).

Tests added

src/lib/mappers/tests/content-item-mapper.test.ts

• New test: "throws when source already has a mapping but a different unmapped target is provided (duplicate mapping attempt)"
• Verifies that ContentItemMapper.addMapping throws 'Aborting a duplicate mapping attempt' when a source content item (ID 10) that is already mapped to a target (ID 20) is subsequently mapped to a different, previously-unmapped target (ID 99). This guards against silent data corruption where one source would end up pointing to multiple targets.

src/lib/pushers/content-pusher/util/tests/get-content-item-types.test.ts

• New test: "item already in linkedSet is not re-added to normalSet when its own loop iteration runs"
• Verifies that when a linked item appears after its parent in the input array, its own loop iteration does not incorrectly promote it back into normalContentItems. The test asserts that linkedItem appears exactly once in linkedContentItems and is absent from normalContentItems.

Related

• PR Fix duplicate content mappings #143 — introduced the guards being tested
• Jira PROD-1439 — duplicate content mapping issue

Test plan

• yarn test src/lib/mappers/tests/content-item-mapper.test.ts — new test passes
• yarn test src/lib/pushers/content-pusher/util/tests/get-content-item-types.test.ts — new test passes
• Full test suite passes (yarn test)

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