Fix vector order lookup with composite ordering

[JKamsker]

Summary

• iterate through each ORDER BY segment when parsing vector sort expressions
• ensure vector order detection handles composite ordering metadata
• add regression coverage ensuring vector index search works with composite ORDER BY chains

Testing

• dotnet test LiteDB.sln --settings tests.runsettings

---

https://chatgpt.com/codex/tasks/task_e_68d33c931ed8832a9233ec6a0f964d40