Enable retries by default (#4454)

Fixes awslabs/aws-sdk-rust#1389
Fixes awslabs/aws-sdk-rust#1393

## Problem

When users create a client with `Config::builder().build()`, retries are
disabled by default. This is confusing because our docs say retries are
enabled by default, and `aws_config::load_from_env()` does enable them.
Users expect retries to just work.

## Solution

Enable `RetryConfig::standard()` and a 3.1s connection timeout by
default for `BehaviorVersion::v2026_01_12()` and later. Older behavior
versions keep the current behavior for backward compatibility.

## Changes

- AWS SDK clients now have `RetryConfig::standard()` enabled by default (3 max attempts) when using `BehaviorVersion >= v2026_01_12()`. Previously, retries were disabled by default unless explicitly configured via `aws_config::load_from_env()`
- All clients now have a 3.1s connection timeout by default when using BehaviorVersion >= v2026_01_12. This prevents indefinite hangs on connection attempts.


### Documentation Updates Needed

The AWS Developer Guide at
https://docs.aws.amazon.com/sdk-for-rust/latest/dg/retries.html needs to
be updated to reflect this change.

## Checklist

- [x] Code changes implemented
- [x] Backward compatibility maintained via BehaviorVersion
- [x] Generated documentation updated
- [x] Tests pass

Author: vcjana

aws/codegen-aws-sdk/src/main/kotlin/software/amazon/smithy/rustsdk/AwsFluentClientDecorator.kt

@@ -60,6 +60,7 @@ class AwsFluentClientDecorator : ClientCodegenDecorator {
                     AwsPresignedFluentBuilderMethod(codegenContext),
                     AwsFluentClientDocs(codegenContext),
                     AwsFluentClientRetryPartition(codegenContext),
+                    AwsFluentClientEnableRetries(codegenContext),
                 ).letIf(codegenContext.serviceShape.id == ShapeId.from("com.amazonaws.s3#AmazonS3")) {
                     it + S3ExpressFluentClientCustomization(codegenContext)
                 },
@@ -199,3 +200,25 @@ private class AwsFluentClientRetryPartition(private val codegenContext: ClientCo
         }
     }
 }
+
+/**
+ * Enables retries by default for AWS SDK clients when awsSdkBuild is true.
+ *
+ * Only applies to real AWS SDK builds (controlled by awsSdkBuild in JSON settings).
+ */
+private class AwsFluentClientEnableRetries(private val codegenContext: ClientCodegenContext) : FluentClientCustomization() {
+    override fun section(section: FluentClientSection): Writable {
+        if (!codegenContext.sdkSettings().awsSdkBuild) {
+            return emptySection
+        }
+
+        return when (section) {
+            is FluentClientSection.CustomizeDefaultPluginParams -> {
+                writable {
+                    rust(".with_is_aws_sdk(true)")
+                }
+            }
+            else -> emptySection
+        }
+    }
+}

aws/codegen-aws-sdk/src/test/kotlin/software/amazon/smithy/rustsdk/HttpChecksumTest.kt

@@ -456,6 +456,7 @@ internal class HttpChecksumTest {
                         .region(Region::from_static("doesntmatter"))
                         .with_test_defaults()
                         .http_client(http_client)
+                        .retry_config(#{RetryConfig}::disabled())
                         .build();
 
                     let client = $moduleName::Client::from_conf(config);
@@ -489,6 +490,7 @@ internal class HttpChecksumTest {
                 "tokio" to CargoDependency.Tokio.toType(),
                 "capture_request" to RuntimeType.captureRequest(rc),
                 "http_1x" to CargoDependency.Http1x.toType(),
+                "RetryConfig" to RuntimeType.smithyTypes(rc).resolve("retry::RetryConfig"),
             )
         }
     }

aws/codegen-aws-sdk/src/test/kotlin/software/amazon/smithy/rustsdk/TimeoutConfigMergingTest.kt

@@ -65,6 +65,7 @@ class TimeoutConfigMergingTest {
                         let operation = Duration::from_secs(4);
                         let http_client = infallible_client_fn(|_req| http::Response::builder().body(SdkBody::empty()).unwrap());
                         let sdk_config = SdkConfig::builder()
+                            .behavior_version(aws_smithy_runtime_api::client::behavior_version::BehaviorVersion::v2026_01_12())
                             .timeout_config(
                                 TimeoutConfig::builder()
                                     .connect_timeout(connect_timeout)
@@ -157,8 +158,6 @@ class TimeoutConfigMergingTest {
                             &TimeoutConfig::builder()
                                 .read_timeout(Duration::from_secs(10))
                                 .connect_timeout(connect_timeout)
-                                .disable_operation_attempt_timeout()
-                                .disable_operation_timeout()
                                 .build(),
                             "read timeout overridden"
                         );