Allow polygon shape to consist of disjoint parts

[achaikou]

Issue
Resolves #14297

Approach
Per user's requirement polygon file may contain multiple disjoint polygons. For seismic, we need keep points that are inside one or more polygons.

Polygon shape is for now used only in seismic and seismic is unreleased yet, so we are free to change it.

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'just rapid-tests')

When applicable

• When screenshots are changed: Review screenshot-PR in ert-testdata,
  merge screenshot-PR in ert-testdata before merging this PR.
• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Add backport label to latest release (format: 'backport release-branch-name')

[Copilot]

🟡 Changes recommended

The new union-based parsing can produce polygons with holes, but the current vertex serialization/re-hydration cannot represent holes, which can lead to incorrect contains() results.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR updates PolygonShapeConfig to support boundary files containing multiple polygons (treated as the union of all polygons), enabling seismic observation filtering against disjoint polygon parts.

Changes:

• Change PolygonShapeConfig.vertices from a single ring to a list of polygon rings (multipolygon representation).
• Update polygon parsing to build a union geometry from multiple polygons and preserve disjoint parts while merging overlaps.
• Expand unit test coverage for disjoint, overlapping, and normalization/simplification behavior for multipolygons, and update seismic QC tests accordingly.

File summaries

File	Description
src/ert/config/_shapes.py	Switch polygon handling to multipolygons; union multiple polygons from file and update containment to work across all parts.
tests/ert/unit_tests/config/test_shapes.py	Update expectations for nested vertex structure and add tests for disjoint/overlapping/normalized multipolygons.
tests/ert/unit_tests/config/test_observation_quality_control.py	Verify seismic QC keeps observations inside any of multiple boundary polygons.
tests/ert/unit_tests/config/test_observation_declaration.py	Update boundary parsing assertions to the new nested vertex representation.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 1
• Review effort level: Lite

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.05%. Comparing base (72b8fd4) to head (190e35c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14376      +/-   ##
==========================================
- Coverage   91.06%   91.05%   -0.01%     
==========================================
  Files         495      495              
  Lines       36139    36146       +7     
==========================================
+ Hits        32910    32913       +3     
- Misses       3229     3233       +4     

Flag	Coverage Δ
cli-tests	35.58% <15.78%> (-0.01%)	⬇️
fuzz	43.68% <15.78%> (-0.01%)	⬇️
gui-tests	58.38% <15.78%> (-0.02%)	⬇️
performance-and-unit-tests	80.99% <100.00%> (-0.01%)	⬇️
test	45.35% <15.78%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/ert/config/_shapes.py	96.92% <100.00%> (+0.37%)	⬆️

... and 1 file with indirect coverage changes

[codspeed-hq]

Merging this PR will not alter performance

✅ 36 untouched benchmarks

---

_{Comparing achaikou:seismic_multipolygon (037798a) with main (72b8fd4)}

[Copilot]

🟡 Changes recommended

The current multipolygon cleaning/parsing path can raise unintended runtime exceptions (and produce nondeterministic equality) due to insufficient geometry-type validation and use of assertions for config validation.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

Suppressed comments (1)

src/ert/config/_shapes.py:103

• Using assert isinstance(..., MultiPolygon) for WKT parsing can raise an AssertionError for user-supplied/serialized shapes (e.g. if wkt is a POLYGON). Since PolygonShapeConfig is a config model, this should fail with a regular exception (or accept POLYGON by wrapping it) rather than an assertion that can be stripped with -O.

    def _polygon(self) -> shapely.MultiPolygon:
        multipolygon = shapely.from_wkt(self.wkt)
        assert isinstance(multipolygon, shapely.MultiPolygon)
        shapely.prepare(multipolygon)
        return multipolygon

• Files reviewed: 4/4 changed files
• Comments generated: 2
• Review effort level: Lite