Fix trait/validator loading with discoverModels

When discoverModels(ClassLoader) is called on a bare Model.assembler()
(not viaModel.assembler(ClassLoader)), the provided ClassLoader is used
for model discovery but not for trait/validator factory creation, so
traits from models that are only on the discoverModels' classpath fall
back to DynamicTrait instead as the TraitFactory instance on the
assembler uses the default classpath.

This commit configures the traitFactory and validatorFactory from
the provided ClassLoader when they have not already been set.

Author: kstich

.changes/next-release/bugfix-55db376e5c6cd40ec745a2f5dbce93d0ea94b9bd.json

@@ -0,0 +1,7 @@
+{
+  "type": "bugfix",
+  "description": "Fix trait/validator loading with discoverModels",
+  "pull_requests": [
+    "[#3049](https://github.com/smithy-lang/smithy/pull/3049)"
+  ]
+}

smithy-model/src/main/java/software/amazon/smithy/model/loader/ModelAssembler.java

@@ -393,18 +393,28 @@ public ModelAssembler putMetadata(String name, Node value) {
     }
 
     /**
-     * Discovers models by merging in all models returns by {@link ModelDiscovery}
+     * Discovers models by merging in all models returned by {@link ModelDiscovery}
      * manifests using the provided {@code ClassLoader}.
      *
+     * <p>This method also configures the assembler to use the provided
+     * {@code ClassLoader} for discovering trait and validator service
+     * providers if they have not already been explicitly configured.
+     *
      * @param loader Class loader to use to discover models.
      * @return Returns the model assembler.
      */
     public ModelAssembler discoverModels(ClassLoader loader) {
+        if (traitFactory == null) {
+            traitFactory = TraitFactory.createServiceFactory(loader);
+        }
+        if (validatorFactory == null) {
+            validatorFactory = ValidatorFactory.createServiceFactory(loader);
+        }
         return addDiscoveredModels(ModelDiscovery.findModels(loader));
     }
 
     /**
-     * Discovers models by merging in all models returns by {@link ModelDiscovery}
+     * Discovers models by merging in all models returned by {@link ModelDiscovery}
      * manifests using the thread context {@code ClassLoader}.
      *
      * @return Returns the model assembler.

smithy-model/src/test/java/software/amazon/smithy/model/loader/ModelAssemblerTest.java

@@ -641,6 +641,35 @@ public void canLoadTraitFromJarMultipleTimes() {
         assertTrue(reloadedModel.getShape(ShapeId.from("smithy.test#test")).isPresent());
     }
 
+    // Regression test for https://github.com/smithy-lang/smithy/issues/2596.
+    // When discoverModels(ClassLoader) is called without first configuring a
+    // traitFactory, the assembler should use the provided classloader for trait
+    // SPI discovery so that traits resolve to their concrete classes rather
+    // than falling back to DynamicTrait.
+    @Test
+    public void discoverModelsWithClassLoaderConfiguresTraitFactory() {
+        URL jar = getClass().getResource("jar-traits-import.jar");
+        URL file = getClass().getResource("loads-jar-traits.smithy");
+        URLClassLoader urlClassLoader = new URLClassLoader(new URL[] {jar});
+
+        // Use a plain Model.assembler() call (not Model.assembler(classLoader))
+        // to reproduce the scenario where no explicit traitFactory is set.
+        Model model = Model.assembler()
+                .discoverModels(urlClassLoader)
+                .addImport(file)
+                .assemble()
+                .unwrap();
+
+        ShapeId traitId = ShapeId.from("smithy.test#test");
+        assertTrue(model.getShape(traitId).isPresent());
+
+        // The trait definition shape itself should be present and not a DynamicTrait
+        // on the service that uses it.
+        Shape weatherService = model.expectShape(ShapeId.from("ns.test#Weather"));
+        assertTrue(weatherService.hasTrait(traitId));
+        assertThat(weatherService.findTrait(traitId).get(), not(instanceOf(DynamicTrait.class)));
+    }
+
     @Test
     public void canDisableValidation() {
         String document = "namespace foo.baz\n"