feat(dimensional): add semester and passing_grade to dim_course_run; partially migrate marts__micromasters_dedp_exam_grades

- Add micromasters_examruns CTE to dim_course_run sourced from
  stg__micromasters__app__postgres__exams_examrun; left-join to
  mitxonline_courseruns on courserun_readable_id = examrun_readable_id
  to populate semester and passing_grade for proctored exam runs.
- Add nullable semester (varchar) and passing_grade (double) columns to
  all non-mitxonline platform CTEs and thread through to final SELECT
  and records_to_expire for SCD2 correctness.
- Include semester and passing_grade in the incremental not-exists
  change-detection predicate so exam run config corrections trigger
  new SCD2 records.
- Rewrite MITxOnline branch of marts__micromasters_dedp_exam_grades to
  use tfact_grade + dim_course_run + dim_user instead of
  int__mitxonline__proctored_exam_grades and int__micromasters__users.
- MicroMasters branch retained on int__micromasters__dedp_proctored_exam_grades
  pending MicroMasters being added to tfact_grade (epic #2072).
- user_micromasters_email cast to NULL in MITxOnline branch pending
  dim_user exposing platform-specific emails (epic #2072).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: quazi-h