Cs/ccct 2283 coverage area calculation update

[Charl1996]

Product Description

When a work area is excluded from a microplanning opportunity, it is now consistently treated as non-existent across all coverage calculations and exports:

• The work area group's building count no longer includes buildings from
  excluded work areas.
• The CSV download omits excluded work areas entirely.
• The expected_visit_count on an excluded work area is set to zero at
  the time of exclusion.

Technical Summary

Link to ticket here

This is a release path 3 feature — Experiments & early stage iterations of
product area

Three related gaps were closed to make excluded work areas invisible to
coverage-area calculations:

• WorkAreaGroup.building_count (models.py): The cached_property now
  filters out WorkAreaStatus.EXCLUDED work areas before summing
  building_count, so group-level totals reflect only active areas.
• download_work_areas view (views.py): The base queryset for the CSV
  export now excludes work areas with EXCLUDED status before passing to
  WorkAreaMapFilterSet, so excluded areas never appear in downloads.
• exclude_work_areas view (views.py): When a work area is excluded,
  expected_visit_count is set to 0 and included in update_fields,
  ensuring the visit target is zeroed out at exclusion time.

Safety Assurance

Safety story

Changes are isolated to read paths (queryset filtering, property aggregation)
and a single extra field write during the existing exclusion flow. No
migrations are required. Excluded work areas already had their
work_area_group nulled out; this PR extends that same cleanup logic to
expected_visit_count and to the two remaining read surfaces. Existing
excluded records will have their expected_visit_count corrected the next
time they are excluded (no backfill needed for historical data since this is
an early-stage feature).

Automated test coverage

• TestWorkAreaGroupBuildingCount (tests/test_models.py) — three
  cases: sums non-excluded areas, ignores excluded areas, returns zero when all
  excluded.
• TestDownloadWorkAreas.test_excludes_excluded_work_areas
  (tests/test_views.py) — verifies excluded work areas are absent from the
  CSV download.
• TestExcludeWorkAreas (tests/test_views.py) — extended with an
  assertion that expected_visit_count == 0 after exclusion.

QA Plan

N.A

Labels & Review

• The set of people pinged as reviewers is appropriate for the level of
  risk of the change

[pxwxnvermx]

Couple of questions regarding clustering.

1. Do the excluded work areas still remain a part of the work area group that was assigned to them during clustering?
2. Can a work area be excluded before the clustering is run? If yes, should that work area be excluded from the clustering?

[Charl1996]

@pxwxnvermx

Do the excluded work areas still remain a part of the work area group that was assigned to them during clustering?

I think so? Let me confirm.

Can a work area be excluded before the clustering is run? If yes, should that work area be excluded from the clustering?

Not as far as I'm aware, but I'll check as well.

[Charl1996]

@pxwxnvermx

Can a work area be excluded before the clustering is run? If yes, should that work area be excluded from the clustering?

See 9460268

Do the excluded work areas still remain a part of the work area group that was assigned to them during clustering?

Confirmed for now it will be.

[zandre-eng]

Minor nit: Since these values aren't being tested anywhere these fields can safely be removed as they get given defaults by the factory.