openai: Don't send prompt_cache_key for OpenAI-compatible models (#36231)

Some APIs fail when they get this parameter

Closes #36215

Release Notes:

- Fixed OpenAI-compatible providers that don't support prompt caching
and/or reasoning

crates/vercel/src/vercel.rs

@@ -71,4 +71,8 @@ impl Model {
             Model::Custom { .. } => false,
         }
     }
+
+    pub fn supports_prompt_cache_key(&self) -> bool {
+        false
+    }
 }