copilot: Remove PromptTokensDetails from Usage struct (#33029)

Closes #33024

Release Notes:

- Removed `PromptTokensDetails` from `Usage` as Gemini no longer
supplies cached token data for copilot.

crates/copilot/src/copilot_chat.rs

@@ -318,15 +318,9 @@ pub struct ResponseEvent {
 pub struct Usage {
     pub completion_tokens: u64,
     pub prompt_tokens: u64,
-    pub prompt_tokens_details: PromptTokensDetails,
     pub total_tokens: u64,
 }
 
-#[derive(Deserialize, Debug)]
-pub struct PromptTokensDetails {
-    pub cached_tokens: u64,
-}
-
 #[derive(Debug, Deserialize)]
 pub struct ResponseChoice {
     pub index: usize,