open_ai: Send prompt_cache_key to improve caching (#36065)

Release Notes:

- N/A

Co-authored-by: Michael Sloan <mgsloan@gmail.com>

crates/language_models/src/provider/open_ai.rs

@@ -455,6 +455,7 @@ pub fn into_open_ai(
         } else {
             None
         },
+        prompt_cache_key: request.thread_id,
         tools: request
             .tools
             .into_iter()

crates/open_ai/src/open_ai.rs

@@ -244,6 +244,8 @@ pub struct Request {
     pub parallel_tool_calls: Option<bool>,
     #[serde(default, skip_serializing_if = "Vec::is_empty")]
     pub tools: Vec<ToolDefinition>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub prompt_cache_key: Option<String>,
 }
 
 #[derive(Debug, Serialize, Deserialize)]