fix: allow deleting Flow Model despite Session/Agent references

[ShrihariMahabal]

Summary

Sessions and Agents keep a historical/required link to a Flow Model, which blocked deleting the model. Adds them to ignore_links_on_delete so the link check no longer blocks it.

[greptile-apps]

Confidence Score: 5/5

Safe to merge — adding Flow Session to ignore_links_on_delete is a minimal, targeted change that only relaxes the link-check for historical session references.

The one-line change is narrowly scoped: sessions hold read-only historical bookkeeping links to Flow Model, so skipping the link check on delete is correct and carries no operational risk.

No files require special attention.

_{Reviews (2): Last reviewed commit: "fix: keep Flow Agent link check, only ex..." | Re-trigger Greptile}