[app-server] feat: add command to generate json schema (#6406)

Add a `codex generate-json-schema` command for generating a JSON schema
bundle of app-server types, analogous to the existing `codex
generate-ts` command for Typescript.

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

@@ -666,6 +666,8 @@ fn ts_files_in_recursive(dir: &Path) -> Result<Vec<PathBuf>> {
     Ok(files)
 }
 
+/// Generate an index.ts file that re-exports all generated types.
+/// This allows consumers to import all types from a single file.
 fn generate_index_ts(out_dir: &Path) -> Result<PathBuf> {
     let mut entries: Vec<String> = Vec::new();
     let mut stems: Vec<String> = ts_files_in(out_dir)?