Fix OpenAI temperature handling (#526)

* Fix OpenAI temperature handling

* Simplify temperature default

* Inline temperature default

* Inline temperature default (one-line)

* Fix type error in temperature handling

Use walrus operator to properly narrow type from float | None to float,
while still allowing temperature=0 and defaulting to 1.0 when None.

---------

Co-authored-by: Cursor Bot <bot@cursor.com>

Author: corbt

src/art/tinker/service.py

@@ -339,7 +339,9 @@ async def chat_completions(
                     max_tokens=body.get("max_completion_tokens")
                     or body.get("max_tokens"),
                     seed=body.get("seed"),
-                    temperature=body.get("temperature") or 1.0,
+                    temperature=t
+                    if (t := body.get("temperature")) is not None
+                    else 1.0,
                     top_k=body.get("top_k") or -1,
                     top_p=body.get("top_p") or 1.0,
                 ),