fix(graph): parameterize Kuzu isolation timestamps

[Yasou13]

Summary

• replace three test-only Kuzu current timestamp function calls with a shared UTC parameter-binding helper
• keep the real rogue-edge tenant-isolation assertions unchanged
• add focused regression coverage for UTC-aware parameter binding
• align the provider timestamp contract documentation with production behavior

Verification

• tests/test_kuzu_isolation.py: 11 passed
• graph, Kuzu, V4 retrieval, scope/security, and projection tests: 127 passed
• full suite: 1933 passed
• Ruff, layer checks, mypy, compile/import, dependency and lock validation: passed

Final SHA: 9b63f74

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