[FF cleanup] remove resolve_vulnerability_send_finding_description flag

What does this MR do and why?

Context

We had stopped sending the description of a vulnerability finding when this feature was first launched, in order to save on tokens.

With newer models, this token constraint is no longer a worry, so we want to start sending the finding description in the LLM prompt again.

This change

We added a derisk FF just to be really really sure the description wouldn't result in LLM errors.

The flag has been globally enabled, and we see no error rate difference in the dashboard.

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closes: #537615 (closed)
EE: true
Changelog: removed