feat: make upsert_edge type-aware with property update support

[anonenity]

Summary

• upsert_edge now checks for edges of the same relationship type before deciding to create or update, instead of checking for any edge between two nodes
• Existing same-type edges have their properties updated via SET (merge semantics), preserving properties not included in the update
• Applied to both Rust and Python bindings

Problem

Previously, upsert_edge called has_edge(source, target) which matched any edge type. This caused two bugs:

1. Could not create multiple relationship types between the same node pair -- the second upsert_edge would see the first edge and silently no-op
2. No way to update edge properties through the Graph API -- existing edges were never modified

[dylanbstorey]

Hey @anonenity — nice fix, the type-aware MERGE approach is the right call. I pushed a commit with three failing tests that highlight an API gap opened up by this change:

• test_get_edge_by_type — with two edge types between the same pair, get_edge has no way to request a specific one
• test_delete_edge_by_type — delete_edge nukes all edges between a pair instead of just the specified type
• test_has_edge_by_type — has_edge can't check for a specific relationship type

All three fail with TypeError. Now that upsert_edge supports multiple typed edges between the same nodes, the rest of the edge API needs to keep up. Can you make these tests pass?

[anonenity]

Hey @dylanbstorey, thanks! All three tests passing now. Agreed, type-aware MERGE is right here. Added optional rel_type to get_edge, has_edge, and delete_edge across both bindings.