Fix duplication in methpaths when directed=False

[yu-ta-sato]

Description

Fix duplicated metapaths in undirected mode for both add_metapaths_by_weight() and add_metapaths().

For add_metapaths_by_weight(), undirected endpoint pairs are now canonicalized and deduplicated before new edges are materialized, so mirrored pairs like (u, v) and (v, u) no longer both appear.

For add_metapaths(), undirected path aggregation now canonicalizes mirrored traversals before grouping, removing duplicate reverse-orientation metapaths while preserving distinct paths between the same endpoints.

Added regression tests covering:

• undirected weighted metapaths from tuple input
• undirected weighted metapaths in NetworkX MultiGraph output
• undirected structural metapaths with mirrored traversals

Related Issues

NA

Checklist

• I have read the Contributing Guide.
• I have updated the documentation, if necessary.
• I have added tests to cover my changes.
• All new and existing tests passed.
• Pre-commit checks passed locally.

[codecov]

Codecov Report

❌ Patch coverage is 97.56098% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
city2graph/metapath.py	97.56%	1 Missing ⚠️

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