test: add jsonrpc payload coverage (#420)

Summary:
- Extract JSONRPC payload parse/dispatch into a small testable helper
while preserving `start_server` behavior.
- Add tests for valid payload dispatch and malformed JSON rejection
without handler invocation.
- Covers the first `pet-jsonrpc` slice from the #389 coverage plan.

Validation:
- cargo test -p pet-jsonrpc
- cargo fmt --all
- cargo clippy --all -- -D warnings

Refs #389

Author: karthiknadig

crates/pet-jsonrpc/src/server.rs

@@ -120,11 +120,8 @@ pub fn start_server<C>(handlers: &HandlersKeyedByMethodName<C>) -> ! {
                             Ok(_) => {
                                 let request =
                                     String::from_utf8_lossy(&buffer[..content_length]).to_string();
-                                match serde_json::from_str(&request) {
-                                    Ok(request) => handlers.handle_request(request),
-                                    Err(err) => {
-                                        eprint!("Failed to parse LINE: {request}, {err:?}")
-                                    }
+                                if let Err(err) = handle_payload(handlers, &request) {
+                                    eprint!("Failed to parse LINE: {request}, {err:?}")
                                 }
                                 continue;
                             }
@@ -141,6 +138,15 @@ pub fn start_server<C>(handlers: &HandlersKeyedByMethodName<C>) -> ! {
     }
 }
 
+fn handle_payload<C>(
+    handlers: &HandlersKeyedByMethodName<C>,
+    payload: &str,
+) -> Result<(), serde_json::Error> {
+    let request = serde_json::from_str(payload)?;
+    handlers.handle_request(request);
+    Ok(())
+}
+
 /// Parses the content length from the given line.
 fn get_content_length(line: &str) -> Result<usize, String> {
     let line = line.trim();
@@ -237,6 +243,44 @@ mod tests {
         assert_eq!(*context.notification.lock().unwrap(), Some(json!(["item"])));
     }
 
+    #[test]
+    fn handle_payload_routes_valid_jsonrpc_payload() {
+        let context = Arc::new(TestContext::default());
+        let mut handlers = HandlersKeyedByMethodName::new(context.clone());
+        handlers.add_request_handler("request/method", |context, id, params| {
+            *context.request.lock().unwrap() = Some((id, params));
+        });
+
+        handle_payload(
+            &handlers,
+            r#"{"jsonrpc":"2.0","id":9,"method":"request/method","params":{"ok":true}}"#,
+        )
+        .unwrap();
+
+        assert_eq!(
+            *context.request.lock().unwrap(),
+            Some((9, json!({ "ok": true })))
+        );
+    }
+
+    #[test]
+    fn handle_payload_rejects_malformed_json_without_dispatching() {
+        let context = Arc::new(TestContext::default());
+        let mut handlers = HandlersKeyedByMethodName::new(context.clone());
+        handlers.add_request_handler("request/method", |context, id, params| {
+            *context.request.lock().unwrap() = Some((id, params));
+        });
+
+        let error = handle_payload(
+            &handlers,
+            r#"{"jsonrpc":"2.0","id":9,"method":"request/method","params": "#,
+        )
+        .unwrap_err();
+
+        assert!(error.is_eof());
+        assert!(context.request.lock().unwrap().is_none());
+    }
+
     #[test]
     fn handle_request_reports_unknown_methods_without_invoking_known_handlers() {
         let context = Arc::new(TestContext::default());