Implement protobuf enum and message generation

Add `write_enum` and `write_message` to generate Rust enums and structs
from protobuf descriptors. Update the generator tests to support the
updated `generate_rust_code` signature.

tests/build_generated_code.rs

@@ -30,8 +30,8 @@ fn test_generated_code_builds() {
     let set = roto::proto_gen::google::protobuf::descriptor::FileDescriptorSet::new(&set_buf)
         .expect("Failed to create FileDescriptorSet");
 
-    let generated_code = roto::generator::generate_rust_code(&set);
-    assert!(!generated_code.is_empty(), "Generated code should not be empty");
+    let generated_files = roto::generator::generate_rust_code(&set, None, false);
+    assert!(!generated_files.is_empty(), "Generated code should not be empty");
 
     // 2. Setup a temporary Cargo project to verify the code builds
     let root = std::env::current_dir().expect("Failed to get current directory");
@@ -62,7 +62,12 @@ fn test_generated_code_builds() {
     // 4. Write the generated code to src/lib.rs
     // The generated code uses `use crate::{...}`, but it's now in a separate crate.
     // Replace `crate` with `roto` to reference the types in the dependency.
-    let final_code = generated_code.replace("use crate::", "use roto::");
+    let mut all_code = String::new();
+    for (_, content) in generated_files {
+        all_code.push_str(&content);
+        all_code.push_str("\n");
+    }
+    let final_code = all_code.replace("use crate::", "use roto::");
     let lib_path = temp_project_dir.join("src/lib.rs");
     fs::write(lib_path, final_code).expect("Failed to write generated code to src/lib.rs");