Add changelog for enabling retries by default (#4454) (#4492)

Adds missing changelog entry for PR #4454 which enabled retries by default for AWS SDK clients starting with
`BehaviorVersion::v2026_01_12()`.

Author: vcjana

.changelog/1769183956.md

@@ -0,0 +1,29 @@
+---
+applies_to:
+- client
+- aws-sdk-rust
+authors:
+- vcjana
+references:
+- smithy-rs#4454
+- smithy-rs#4467
+- aws-sdk-rust#1389
+breaking: true
+new_feature: true
+bug_fix: false
+---
+Enable retries by default for AWS SDK clients using `BehaviorVersion::v2026_01_12()` or later.
+
+Previously, retries were only enabled when constructing clients via `aws_config::load_from_env()`. Clients constructed directly using `Client::from_conf()` did not have retries enabled, which was inconsistent with AWS SDK behavior in other languages.
+
+This change affects AWS SDK clients constructed with `Client::from_conf()` when using `BehaviorVersion::v2026_01_12()` or `BehaviorVersion::latest()`. Generic Smithy clients (non-AWS) are not affected.
+
+To disable retries:
+```rust
+let config = aws_sdk_s3::Config::builder()
+    .retry_config(RetryConfig::disabled())
+    // ...
+    .build();
+```
+
+For more context, see the [discussion on retry behavior](https://github.com/smithy-lang/smithy-rs/discussions/4466).