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/parse_command.rs

@@ -18,11 +18,14 @@ pub enum ParsedCommand {
     },
     ListFiles {
         cmd: String,
+        #[ts(optional = nullable)]
         path: Option<String>,
     },
     Search {
         cmd: String,
+        #[ts(optional = nullable)]
         query: Option<String>,
+        #[ts(optional = nullable)]
         path: Option<String>,
     },
     Unknown {