Fix generated code and update example integration

Resolve `Result` ambiguity and lifetime issues in generated services.
Use `file_stem` for proto filenames. Make `StatusBody` public in
`roto-tonic` and update the `hello_world` build process.

examples/hello_world/build.rs

@@ -4,9 +4,9 @@ fn main() {
     let dest_path = std::path::Path::new(&out_dir).join("hello.rs");
 
     // Find the protoc-gen-roto binary
-    // In a real scenario, this should be passed as an environment variable or found in PATH
-    // For this example, we'll try to find it in the target directory
-    let target_dir = std::env::current_dir().unwrap().join("../../target/debug");
+    // Since we added roto-codegen to build-dependencies, it will be built.
+    let manifest_dir = std::env::var("CARGO_MANIFEST_DIR").unwrap();
+    let target_dir = std::path::Path::new(&manifest_dir).join("../../target/debug");
     let plugin_path = target_dir.join("protoc-gen-roto");
 
     if !plugin_path.exists() {