fix(dimensional): platform-scope MicroMasters program_fk lookup in tfact_enrollment

[blarghmatey]

What are the relevant tickets?

Closes #2378

Description (What does it do?)

tfact_enrollment.sql's micromasters_program_lookup CTE enriched course-scope enrollments with a MicroMasters program_fk by joining on courserun_readable_id alone, with no platform constraint. That meant:

• Any platform's enrollment sharing a MM-linked courserun_readable_id (not just the edxorg/mitxonline enrollments that MM courses actually run on) could inherit a MicroMasters program_fk.
• A course run reused across two different MicroMasters programs would fan out the left join, silently masked by the model's final defensive dedup (which just picks an arbitrary winning row).
• _fact_tables.yml documented program_fk as null for course enrollments except the mitxpro program-purchase case, contradicting the MM enrichment behavior the code already relied on.

Fix:

• Platform-scope the lookup by carrying the enrollment's own platform value (mapped from the source edX.org/MITx Online strings to the edxorg/mitxonline codes used elsewhere in the model) and joining on (courserun_readable_id, platform) instead of courserun_readable_id alone.
• Dedupe micromasters_program_lookup to one row per (courserun_readable_id, platform) via row_number() (Trino has no QUALIFY, consistent with the existing dedup pattern later in the same model) so a course run belonging to two MM programs can't fan out the enrollment it's joined against.
• Updated the program_fk column description in _fact_tables.yml to document the MicroMasters enrichment path.

How can this be tested?

• dbt parse and dbt compile -s tfact_enrollment both succeed against the local duckdb target; the compiled SQL confirms the Jinja var() calls resolve to the correct source platform strings (edX.org, MITx Online) used in the case mapping.
• Existing relationships test on program_fk → dim_program.program_pk and the dbt_utils.unique_combination_of_columns test on (enrollment_id, platform, enrollment_type) continue to guard the model's grain; a full run against the QA warehouse is recommended before merge to confirm no unexpected program_fk population changes for edge-case platforms.

Additional Context

Found via the dbt warehouse semantic-accuracy audit (dimensional-layer correctness epic). Related identity-collapse issue in the same audit: MicroMasters certificates in tfact_certificate also dual-source from edxorg with a separate defensive-dedup gap (not addressed by this PR).

[Copilot]

Pull request overview

This PR fixes a dimensional-layer correctness issue in tfact_enrollment where MicroMasters program_fk enrichment could leak across platforms by joining on courserun_readable_id alone. It aligns the model behavior with the platform-pairing rule described in the issue/PR description and updates the schema docs to reflect the intended enrichment behavior.

Changes:

• Platform-scopes the MicroMasters program lookup by joining on (courserun_readable_id, platform) rather than courserun_readable_id alone.
• Pre-dedupes the MicroMasters lookup to a single row per (courserun_readable_id, platform) using row_number() (Trino-compatible pattern).
• Updates _fact_tables.yml documentation for program_fk to include the MicroMasters enrichment behavior for edxorg/mitxonline course enrollments.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/ol_dbt/models/dimensional/tfact_enrollment.sql	Adds platform-aware MicroMasters lookup + Trino-friendly pre-dedup and joins it on both courserun and platform to prevent cross-platform leakage/fanout.
src/ol_dbt/models/dimensional/_fact_tables.yml	Updates program_fk column documentation to reflect MicroMasters-based population for edxorg/mitxonline course enrollments.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[github-actions]

🔎 ol-dbt impact — column-level blast radius

✅ No column-level downstream impact detected for the changed models.

_{Posted by ol-dbt impact (annotate-only — does not block merge).}

[rachellougee]

LGTM. The changes are mostly preventive/defensive - I verified against production: no enrollment currently has a cross-platform MM program_fk, and no course run currently maps to two MM programs. So no full refresh needed.