fix #15325: guard Langfuse optional params against cycles

[ThomaciousD]

• add a cycle-aware sanitizer that walks optional_params once and returns "" whenever a previously-seen object shows up again
• remove the old best-effort str() coercion loop so every nested value is normalised through the sanitizer before Langfuse logging
• leave the rest of the Langfuse logging flow unchanged; usage/cost metadata still passes through as before

[vercel]

Someone is attempting to deploy a commit to the CLERKIEAI Team on Vercel.

A member of the Team first needs to authorize it.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

Thomas Dandelot seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[krrishdholakia]

add testing please