fix: move account struct to app-server-protocol and use camelCase (#5829)

Makes sense to move this struct to `app-server-protocol/` since we want
to serialize as camelCase, but we don't for structs defined in
`protocol/`

It was:
```
export type Account = { "type": "ApiKey", api_key: string, } | { "type": "chatgpt", email: string | null, plan_type: PlanType, };
```

But we want:
```
export type Account = { "type": "apiKey", apiKey: string, } | { "type": "chatgpt", email: string | null, planType: PlanType, };
```

codex-rs/protocol/src/account.rs

@@ -18,18 +18,3 @@ pub enum PlanType {
     #[serde(other)]
     Unknown,
 }
-
-#[derive(Debug, Clone, PartialEq, Deserialize, Serialize, JsonSchema, TS)]
-#[serde(tag = "type")]
-#[ts(tag = "type")]
-pub enum Account {
-    ApiKey {
-        api_key: String,
-    },
-    #[serde(rename = "chatgpt")]
-    #[ts(rename = "chatgpt")]
-    ChatGpt {
-        email: Option<String>,
-        plan_type: PlanType,
-    },
-}