add back previous fallback behavior to custom v1 resolvers

codegen/smithy-aws-go-codegen/src/main/java/software/amazon/smithy/aws/go/codegen/AddAwsConfigFields.java

@@ -183,6 +183,13 @@ public class AddAwsConfigFields implements GoIntegration {
                     .documentation("API stack mutators")
                     .generatedOnClient(false)
                     .build(),
+            AwsConfigField.builder()
+                    .name(ENDPOINT_RESOLVER_CONFIG_NAME)
+                    .type(getAwsCoreSymbol("EndpointResolver"))
+                    .generatedOnClient(false)
+                    .awsResolveFunction(SymbolUtils.createValueSymbolBuilder(RESOLVE_AWS_CONFIG_ENDPOINT_RESOLVER)
+                            .build())
+                    .build(),
             AwsConfigField.builder()
                     .name(LOGGER_CONFIG_NAME)
                     .type(getAwsCoreSymbol("Logger"))
@@ -255,6 +262,7 @@ private void writeAwsDefaultResolvers(GoWriter writer) {
         writeHttpClientResolver(writer);
         writeRetryerResolvers(writer);
         writeRetryMaxAttemptsFinalizeResolver(writer);
+        writeAwsConfigEndpointResolver(writer);
     }
 
     private void writerAwsDefaultResolversTests(GoWriter writer) {
@@ -573,6 +581,25 @@ private void writeHttpClientResolver(GoWriter writer) {
         writer.popState();
     }
 
+    private void writeAwsConfigEndpointResolver(GoWriter writer) {
+        writer.pushState();
+        writer.putContext("resolverName", RESOLVE_AWS_CONFIG_ENDPOINT_RESOLVER);
+        writer.putContext("clientOption", ENDPOINT_RESOLVER_CONFIG_NAME);
+        writer.putContext("wrapperHelper", EndpointGenerator.AWS_ENDPOINT_RESOLVER_HELPER);
+        writer.putContext("awsResolver", ENDPOINT_RESOLVER_CONFIG_NAME);
+        writer.putContext("awsResolverWithOptions", AWS_ENDPOINT_RESOLVER_WITH_OPTIONS);
+        writer.putContext("newResolver", EndpointGenerator.RESOLVER_CONSTRUCTOR_NAME);
+        writer.write("""
+                func $resolverName:L(cfg aws.Config, o *Options) {
+                    if cfg.$awsResolver:L == nil && cfg.$awsResolverWithOptions:L == nil {
+                        return
+                    }
+                    o.$clientOption:L = $wrapperHelper:L(cfg.$awsResolver:L, cfg.$awsResolverWithOptions:L, $newResolver:L())
+                }
+                """);
+        writer.popState();
+    }
+
     @Override
     public List<RuntimeClientPlugin> getClientPlugins() {
         List<RuntimeClientPlugin> plugins = new ArrayList<>();

codegen/smithy-aws-go-codegen/src/main/java/software/amazon/smithy/aws/go/codegen/EndpointGenerator.java

@@ -625,7 +625,6 @@ private void generatePublicResolverTypes(GoWriter writer) {
             writer.write("return fn(region, options)");
         });
 
-
         // Generate EndpointResolverFromURL helper
         writer.writeDocs(String.format("%s returns an EndpointResolver configured using the provided endpoint url. "
                                        + "By default, the resolved endpoint resolver uses the client region as signing region, and  "