fix: aggregate undirected SQL degrees

[j6k4m8]

Summary

• aggregate both endpoint columns for undirected SQL degree queries
• return matching in-degree and out-degree results for undirected graphs
• include explicitly requested isolated nodes with degree zero
• preserve caller key types in bulk degree results

[codspeed-hq]

Merging this PR will not alter performance

✅ 17 untouched benchmarks

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_{Comparing fix/sql-undirected-degrees-81 (67cd31f) with master (f6d99e9)}