fix: return undirected DataFrame predecessors

[j6k4m8]

Summary

• return the endpoint opposite the queried node for undirected predecessors
• apply the same endpoint selection in metadata mode
• restore DataFrame coverage in the shared predecessor contract
• cover both stored edge orientations and metadata values

[codspeed-hq]

Merging this PR will not alter performance

✅ 17 untouched benchmarks

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_{Comparing fix/dataframe-undirected-predecessors-77 (972054b) with master (b4d3c33)}