Investigate missing DTXSID field in chemi_cluster()

[seanthimons]

Context

Inline TODO at R/chemi_cluster.R:77,86 — some compounds don't return a DTXSID field from the clustering endpoint. The DTXSID field was removed from processing as a workaround, but the root cause is unclear.

Tasks

• Identify which compounds trigger the missing DTXSID
• Determine if this is an API-side issue or data gap
• Implement proper handling (conditional inclusion vs. hard removal)