Fix duplicate posts when re-importing content without slugs

[faisalahammad]

Resolves #8066

Proposed Changes

Content import duplicates posts when a source CSV row has no slug. The import only matched existing posts by slug, so a row with an empty slug was always treated as new and inserted again on re-import. One reported run of 1,320 lesson rows produced 1,342 lesson posts.

The fix persists the source id as a private post meta (_sensei_import_id) on every imported post. When the slug lookup misses, the import now falls back to that meta to find the existing post, so re-importing the same CSV updates the same posts instead of creating new ones. Rows with no slug and no id still cannot be matched and are documented as a known limit.

Screenshots

No visual change.

Testing Instructions

• Import a CSV that contains a row with an empty slug and a non-empty id.
• Import the same CSV a second time.
• Confirm the post count does not double: the slug-less rows resolve to their existing posts.

New/Updated Hooks

None.

Deprecated Code

None.

Changelog entry

• Automatically create a changelog entry from the details below.

Changelog Entry Details

Significance

• Patch - Backward-compatible bug fixes

Type

• Fixed - Fixes a bug

Message

Fix duplicate posts when re-importing content with empty slug columns by persisting the source import id on each imported post.

[faisalahammad]

CI status update

Psalm (8.2) fixed. The import-id refactor in class-sensei-import-model.php surfaced five static-analysis issues (all false positives on unit-tested, runtime-correct code), baselined in config/psalm/psalm-baseline.xml rather than worked around (./vendor/bin/psalm now exits clean).

Two checks remain, unfixable here:

• Check Milestone — PR has no milestone; I lack push access to Automattic/sensei, so a maintainer must assign one.
• Expose Built Artifact — HTTP 403 uploading to the ci-playground-artifacts release. Fork PRs get read-only GITHUB_TOKEN, so release asset upload is denied; only passes for org-repo branches.