Fix Gremlin predecessor and edge traversal behavior

[j6k4m8]

Summary

• scope predecessor traversals to the requested vertex and return predecessor IDs
• return edge metadata dictionaries, including empty metadata, with consistent missing-edge errors
• make edge creation and update paths use the same existence contract
• add mocked traversal regression coverage without external resources

[codspeed-hq]

Merging this PR will not alter performance

✅ 17 untouched benchmarks

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_{Comparing fix/gremlin-correctness-78-79 (f9b3a9a) with master (07959fb)}