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

@@ -30,6 +30,7 @@ pub enum JSONRPCMessage {
 
 /// A request that expects a response.
 #[derive(Debug, Clone, PartialEq, Deserialize, Serialize, JsonSchema, TS)]
+#[ts(optional_fields = nullable)]
 pub struct JSONRPCRequest {
     pub id: RequestId,
     pub method: String,
@@ -39,6 +40,7 @@ pub struct JSONRPCRequest {
 
 /// A notification which does not expect a response.
 #[derive(Debug, Clone, PartialEq, Deserialize, Serialize, JsonSchema, TS)]
+#[ts(optional_fields = nullable)]
 pub struct JSONRPCNotification {
     pub method: String,
     #[serde(default, skip_serializing_if = "Option::is_none")]
@@ -60,6 +62,7 @@ pub struct JSONRPCError {
 }
 
 #[derive(Debug, Clone, PartialEq, Deserialize, Serialize, JsonSchema, TS)]
+#[ts(optional_fields = nullable)]
 pub struct JSONRPCErrorError {
     pub code: i64,
     #[serde(default, skip_serializing_if = "Option::is_none")]