refactor: Extract geo+vectors fetch and merge into dedicated functions

[dshkol]

Summary

Extract the complex hybrid geo+vectors logic (~70 lines) from get_census() into two dedicated helper functions.

Changes

Added two new functions:

_fetch_census_with_geometry_and_vectors()

• Handles the CensusMapper API quirk where geo.geojson doesn't return vector data properly
• Makes separate calls to geo.geojson and data.csv endpoints
• Orchestrates the merge of geometry and vector data

_merge_geo_and_csv_results()

• Merges GeoDataFrame with CSV DataFrame on geographic identifier
• Detects common merge keys (GeoUID, id, rgid) automatically
• Falls back to index-based merge if no common key found
• Handles duplicate key cleanup after merge

Simplified get_census()

# Before: 70+ lines of inline merge logic
# After:
if geo_format == "geopandas" and vectors:
    result = _fetch_census_with_geometry_and_vectors(
        base_url, request_data, resolution, vectors, labels
    )

Why

• Readability: get_census() main flow is now much simpler
• Testability: Merge logic can be unit tested in isolation
• Single responsibility: Each function has one clear purpose
• Documentation: Detailed docstrings explain the API quirk

Test plan

• All existing tests pass (17 total)
• Added test_merge_on_geoid_key: verifies key-based merge
• Added test_merge_fallback_by_index: verifies index fallback

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