Use nanosecond-based requestid for convert operations

This fixes flaky tests where multiple collateralized_convert or convert
operations created within the same second would get the same requestid
(based on seconds since epoch), causing uniqueness constraint violations.

Using time.time_ns() % (2**32) ensures unique IDs even when operations
are created in quick succession, while still fitting in the uint32_t
requestid field.

🤖 Generated with [Claude Code](https://claude.com/claude-code

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Co-Authored-By: Claude <noreply@anthropic.com>