Bugfix/54 issue with inner outer holes sequence

[rapkin]

Description

Fixed issue #54 where OSM relations with non-consecutive
role ordering would incorrectly generate multiple outer
polygons instead of a single connected boundary.

Problem: When an OSM multipolygon relation had members
ordered as outer-outer-inner-outer (with outer ways split by
inner ways), the consecutive grouping logic would create
separate outer groups, resulting in fragmented polygons.

Solution: When multiple outer groups exist after consecutive
grouping, the fix attempts to merge all outer lines
together. The merged result is only used if it creates
exactly 1 polygon (indicating the segments actually
connect). This preserves correct behavior for complex cases
like Baarle-Nassau (issue #35) where separate outer groups
are intentional.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring
• Performance improvement
• Test addition or update

Related Issues

Fixes #54

Changes Made

• Added logic in _convert_shapes_to_multipolygon() to detect
  and merge disconnected outer groups when they form a single
  connected polygon
• Added test case test_issue_54_staffordshire_multipolygon
  with real-world OSM data (relation 195444 - Staffordshire
  county boundary)
• Implementation is concise (~15 lines of additional logic)
  and maintains backward compatibility

Testing

• All existing tests pass
• Added new tests for new functionality
• Tested manually (describe below)

Manual Testing Steps:

1. generated geojson with osmtogeojson and visualised
2. generated geojson with current version
3. compared visually

Checklist

• My code follows the code style of this project (ran ruff format and ruff check)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Screenshots (if applicable)

Additional Notes