open_ai: Fix issues with OpenAI compatible APIs (#32982)

Ran into this while adding support for Vercel v0s models:
- The timestamp seems to be returned in Milliseconds instead of seconds
so it breaks the bounds of `created: u32`. We did not use this field
anywhere so just decided to remove it
- Sometimes the `choices` field can be empty when the last chunk comes
in because it only contains `usage`

Release Notes:

- N/A

crates/open_ai/src/open_ai.rs

@@ -385,7 +385,6 @@ pub enum ResponseStreamResult {
 
 #[derive(Serialize, Deserialize, Debug)]
 pub struct ResponseStreamEvent {
-    pub created: u32,
     pub model: String,
     pub choices: Vec<ChoiceDelta>,
     pub usage: Option<Usage>,