feat: add Workflows to ValidationErrorReporter

In order to have proper validation for Workflows I think we will need a
similar setup as DOs: ValidationErrorReporter now has a
`addWorkflowClass` that behaves similarily as `addActorClass`. (This
will need a internal PR in order to work properly).

This will be useful in cases that a user exports a class but doesn't
extend from `WorkflowEntrypoint` (meaning they won't get `ctx` and
`env` and see weird errors).

src/workerd/io/worker.c++

@@ -493,6 +493,7 @@ struct Worker::Impl {
   kj::HashMap<kj::String, api::ExportedHandler> namedHandlers;
   kj::HashMap<kj::String, ActorClassInfo> actorClasses;
   kj::HashMap<kj::String, EntrypointClass> statelessClasses;
+  kj::HashMap<kj::String, EntrypointClass> workflowClasses;
 
   // If set, then any attempt to use this worker shall throw this exception.
   kj::Maybe<kj::Exception> permanentException;
@@ -1690,7 +1691,7 @@ Worker::Worker(kj::Own<const Script> scriptParam,
                               impl->statelessClasses.insert(kj::mv(handler.name), kj::mv(cls));
                               return;
                             } else if (handle == entrypointClasses.workflowEntrypoint) {
-                              impl->statelessClasses.insert(kj::mv(handler.name), kj::mv(cls));
+                              impl->workflowClasses.insert(kj::mv(handler.name), kj::mv(cls));
                               return;
                             }
 
@@ -2206,6 +2207,17 @@ void Worker::Lock::validateHandlers(ValidationErrorReporter& errorReporter) {
               "did not produce a startup-time error.");
         }
       }
+      for (auto& entry: worker.impl->workflowClasses) {
+        KJ_IF_SOME(entrypointName, getEntrypointName(entry.key)) {
+          errorReporter.addWorkflowClass(entrypointName);
+        } else {
+          // Similiar to Durable Objects, Workflow cannot be the default entrypoint (at the time of writing).
+          LOG_PERIODICALLY(ERROR,
+              "Exported Workflow class cannot be the default entrypoint. This doesn't work, but historically "
+              "did not produce a startup-time error.");
+        }
+      }
+
       for (auto& entry: worker.impl->statelessClasses) {
         // We want to report all of the stateless class's members. To do this, we examine its
         // prototype, and it's prototype's prototype, and so on, until we get to Object's

src/workerd/io/worker.h

@@ -86,6 +86,9 @@ class Worker: public kj::AtomicRefcounted {
 
     // Report that the Worker exports a Durable Object class with the given name.
     virtual void addActorClass(kj::StringPtr exportName) = 0;
+
+    // Report that the Worker exports a Workflow class with the given name.
+    virtual void addWorkflowClass(kj::StringPtr exportName) = 0;
   };
 
   class LockType;
@@ -894,6 +897,10 @@ struct SimpleWorkerErrorReporter final: public Worker::ValidationErrorReporter {
     KJ_UNREACHABLE;
   }
 
+  void addWorkflowClass(kj::StringPtr exportName) override {
+    KJ_UNREACHABLE;
+  }
+
   SimpleWorkerErrorReporter() = default;
   KJ_DISALLOW_COPY_AND_MOVE(SimpleWorkerErrorReporter);
   kj::Vector<kj::String> errors;

src/workerd/server/server.c++

@@ -3150,6 +3150,10 @@ kj::Own<Server::Service> Server::makeWorker(kj::StringPtr name,
     void addActorClass(kj::StringPtr exportName) override {
       actorClasses.insert(kj::str(exportName));
     }
+
+    void addWorkflowClass(kj::StringPtr exportName) override {
+      // This is only used for validation and has no runtime implications, at least for now.
+    }
   };
 
   ErrorReporter errorReporter(*this, name);

src/workerd/tests/test-fixture.c++

@@ -224,6 +224,7 @@ struct MockErrorReporter final: public Worker::ValidationErrorReporter {
 
   void addEntrypoint(kj::Maybe<kj::StringPtr> exportName, kj::Array<kj::String> methods) override {}
   void addActorClass(kj::StringPtr exportName) override {}
+  void addWorkflowClass(kj::StringPtr exportName) override {}
 };
 
 inline server::config::Worker::Reader buildConfig(