Implement Anthropic prompt caching (#16274)

Release Notes:

- Adds support for Prompt Caching in Anthropic. For models that support
it this can dramatically lower cost while improving performance.

crates/assistant/src/terminal_inline_assistant.rs

@@ -277,6 +277,7 @@ impl TerminalInlineAssistant {
         messages.push(LanguageModelRequestMessage {
             role: Role::User,
             content: vec![prompt.into()],
+            cache: false,
         });
 
         Ok(LanguageModelRequest {