Update casing of "OpenAI" in identifiers to match Rust conventions (#6940)

This PR updates the casing of "OpenAI" when used in Rust identifiers to
match the [Rust naming
guidelines](https://rust-lang.github.io/api-guidelines/naming.html):

> In `UpperCamelCase`, acronyms and contractions of compound words count
as one word: use `Uuid` rather than `UUID`, `Usize` rather than `USize`
or `Stdin` rather than `StdIn`.

Release Notes:

- N/A

crates/assistant/src/assistant.rs

@@ -7,7 +7,7 @@ mod streaming_diff;
 use ai::providers::open_ai::Role;
 use anyhow::Result;
 pub use assistant_panel::AssistantPanel;
-use assistant_settings::OpenAIModel;
+use assistant_settings::OpenAiModel;
 use chrono::{DateTime, Local};
 use collections::HashMap;
 use fs::Fs;
@@ -68,7 +68,7 @@ struct SavedConversation {
     messages: Vec<SavedMessage>,
     message_metadata: HashMap<MessageId, MessageMetadata>,
     summary: String,
-    model: OpenAIModel,
+    model: OpenAiModel,
 }
 
 impl SavedConversation {