Add missing "nullable" macro to protocol structs that contain optional fields (#5901)

This PR addresses a current hole in the TypeScript code generation for
the API server protocol. Fields that are marked as "Optional<>" in the
Rust code are serialized such that the value is omitted when it is
deserialized — appearing as `undefined`, but the TS type indicates
(incorrectly) that it is always defined but possibly `null`. This can
lead to subtle errors that the TypeScript compiler doesn't catch. The
fix is to include the `#[ts(optional_fields = nullable)]` macro for all
protocol structs that contain one or more `Optional<>` fields.

This PR also includes a new test that validates that all TS protocol
code containing "| null" in its type is marked optional ("?") to catch
cases where `#[ts(optional_fields = nullable)]` is omitted.

codex-rs/protocol/src/custom_prompts.rs

@@ -11,6 +11,7 @@ use ts_rs::TS;
 pub const PROMPTS_CMD_PREFIX: &str = "prompts";
 
 #[derive(Serialize, Deserialize, Debug, Clone, JsonSchema, TS)]
+#[ts(optional_fields = nullable)]
 pub struct CustomPrompt {
     pub name: String,
     pub path: PathBuf,