Fix page_engagement_views_report: platform joins, block category filters, and uniqueness test

[Copilot]

page_engagement_views_report had several correctness issues introduced in the initial implementation.

Changes

• Platform-conditional dim_user joins: Previously only joined on mitxonline_openedx_user_id, silently dropping users from edxorg, mitxpro, and residential. Now uses conditional joins per platform:

  inner join dim_user
      on (
          (page_engagement.platform = 'mitxonline'
              and page_engagement.openedx_user_id = dim_user.mitxonline_openedx_user_id)
          or (page_engagement.platform = 'edxorg'
              and page_engagement.openedx_user_id = dim_user.edxorg_openedx_user_id)
          or (page_engagement.platform = 'mitxpro'
              and page_engagement.openedx_user_id = dim_user.mitxpro_openedx_user_id)
          or (page_engagement.platform = 'residential'
              and page_engagement.openedx_user_id = dim_user.residential_openedx_user_id)
      )

• block_category filters on subsection_blocks and section_blocks: Without these filters, the CTEs included all block types, causing incorrect joins on sequential_block_fk and chapter_block_fk. Now filters sequential and chapter respectively.

• Removed unnecessary GROUP BY from course_runs CTE: int__combined__course_runs already guarantees unique courserun_readable_id values.

• Added uniqueness test: dbt_expectations.expect_compound_columns_to_be_unique on [user_email, courserun_readable_id, unit_title] to enforce expected grain.

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