Fix bounds for multi-member and wrapped GeometryCollections

[jokrasno]

GeoJson.get_bounds() currently considers only the first member of a GeometryCollection. For example, a collection containing points [2, 1] and [40, 30] returns [[1, 2], [1, 2]], excluding the second point. Wrapping a collection in a Feature raises KeyError, and an empty collection raises IndexError.

Recursively traverse Features, FeatureCollections, and all GeometryCollection members in iter_coords. This handles nested and empty collections while preserving null-geometry handling. Follow-up to #1599 and #1633, whose regression covers a single member.

Validation:

• Nine new regression cases assert both layer and map bounds for multiple, nested, and empty collections, directly and inside Features/FeatureCollections. All nine fail before the fix and pass afterward.
• python -B -m pytest tests/test_features.py tests/test_utilities.py tests/test_vector_layers.py -q --tb=short — 116 passed (one existing pytest deprecation warning).
• Changed-file Ruff, Black (Python 3.9 target), and git diff --check pass.
• Tested on Windows in an isolated virtual environment. Browser and notebook suites were not run.