in-memory should be in_memory (64bit#521)

Allowed values are in_memory and 24h

https://platform.openai.com/docs/guides/prompt-caching#configure-per-request

Author: DaleLJefferson

async-openai/src/types/responses/response.rs

@@ -845,7 +845,7 @@ pub enum ReasoningSummary {
 /// The retention policy for the prompt cache.
 #[derive(Debug, Serialize, Deserialize, Clone, Copy, PartialEq, ToSchema)]
 pub enum PromptCacheRetention {
-    #[serde(rename = "in-memory")]
+    #[serde(rename = "in_memory")]
     InMemory,
     #[serde(rename = "24h")]
     Hours24,