fix: revert "[app-server] fix account/read response annotation (#5642)" (#5796)

Revert #5642 because this generates:

```
// GENERATED CODE! DO NOT MODIFY BY HAND!

// This file was generated by [ts-rs](https://github.com/Aleph-Alpha/ts-rs). Do not edit this file manually.

export type GetAccountResponse = Account | null;
```

But `Account` is unknown.

The unique use of `#[ts(export)]` on `GetAccountResponse` is also
suspicious as are the changes to
`codex-rs/app-server-protocol/src/export.rs` since the existing system
has worked fine for quite some time.

Though a pure backout of #5642 puts things in a state where, as the PR
noted, the following does not work:

```
cargo run -p codex-app-server-protocol --bin export -- --out DIR
```

So in addition to the backout, this PR adds:

```rust
#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
#[serde(rename_all = "camelCase")]
pub struct GetAccountResponse {
    pub account: Account,
}
```

and changes `GetAccount.response` as follows:

```diff
-        response: Option<Account>,
+        response: GetAccountResponse,
```

making it consistent with other types.

With this change, I verified that both of the following work:

```
just codex generate-ts --out /tmp/somewhere
cargo run -p codex-app-server-protocol --bin export -- --out /tmp/somewhere-else
```

The generated TypeScript is as follows:

```typescript
// GetAccountResponse.ts
import type { Account } from "./Account";

export type GetAccountResponse = { account: Account, };
```

and

```typescript
// Account.ts
import type { PlanType } from "./PlanType";

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

Though while the inconsistency between `"type": "ApiKey"` and `"type":
"chatgpt"` is quite concerning, I'm not sure if that format is ever
written to disk in any case, but @owenlin0, I would recommend looking
into that.

Also, it appears that the types in `codex-rs/protocol/src/account.rs`
are used exclusively by the `app-server-protocol` crate, so perhaps they
should just be moved there?

codex-rs/app-server-protocol/src/protocol.rs

@@ -225,6 +225,12 @@ client_request_definitions! {
     },
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+pub struct GetAccountResponse {
+    pub account: Account,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Default, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 pub struct InitializeParams {
@@ -535,12 +541,6 @@ pub struct GetAccountRateLimitsResponse {
     pub rate_limits: RateLimitSnapshot,
 }
 
-#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
-#[serde(transparent)]
-#[ts(export)]
-#[ts(type = "Account | null")]
-pub struct GetAccountResponse(#[ts(type = "Account | null")] pub Option<Account>);
-
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 pub struct GetAuthStatusResponse {