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/utils/git/src/lib.rs

@@ -28,6 +28,7 @@ type CommitID = String;
 
 /// Details of a ghost commit created from a repository state.
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize, JsonSchema, TS)]
+#[ts(optional_fields = nullable)]
 pub struct GhostCommit {
     id: CommitID,
     parent: Option<CommitID>,