apply us-east-1 as default region if not set by user in with_test_defaults_v2() (#4312)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->

* #4265
* #4189

## Description
* Adds a new `with_test_defaults_v2()` method to all clients supporting
region configuration. This new method applies `us-east-1` as default
region if not set by user. This allows `aws-smithy-mocks` to work for
non AWS SDK generated clients. This is only set as the default if a user
has not already supplied one.
* NOTE: We are choosing to add a new method here for backwards
compatibility concerns (our own generated endpoint tests break because
of precedence problems between endpoint rules and with_test_defaults).
* Clarify `test-util` feature requirement when using `aws-smithy-mocks`.


## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] For changes to the smithy-rs codegen or runtime crates, I have
created a changelog entry Markdown file in the `.changelog` directory,
specifying "client," "server," or both in the `applies_to` key.
- [x] For changes to the AWS SDK, generated SDK code, or SDK runtime
crates, I have created a changelog entry Markdown file in the
`.changelog` directory, specifying "aws-sdk-rust" in the `applies_to`
key.

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

Author: aajtodd

.changelog/1758554358.md

@@ -0,0 +1,14 @@
+---
+applies_to:
+- client
+- aws-sdk-rust
+authors:
+- aajtodd
+references:
+- smithy-rs#4265
+- smithy-rs#4189
+breaking: false
+new_feature: false
+bug_fix: true
+---
+Adds new `with_test_defaults_v2()` for all clients supporting region configuration which applies `us-east-1` as default region if not set by user. This allows `aws-smithy-mocks` to work for non AWS SDK generated clients. Also clarify `test-util` feature requirement when using `aws-smithy-mocks`.

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

@@ -205,6 +205,17 @@ class RegionProviderConfig(codegenContext: ClientCodegenContext) : ConfigCustomi
                         *codegenScope,
                     )
                 }
+                is ServiceConfig.DefaultForTestsV2 -> {
+                    // this was added later, for backwards compat we only set a default if a region hasn't already been set by user
+                    rustTemplate(
+                        """
+                        if ${section.configBuilderRef}.config.load::<#{Region}>().is_none() {
+                            self.set_region(#{Some}(#{Region}::new("us-east-1")));
+                        }
+                        """,
+                        *codegenScope,
+                    )
+                }
 
                 is ServiceConfig.BuilderFromConfigBag -> {
                     rustTemplate("${section.builder}.set_region(${section.configBag}.load::<#{Region}>().cloned());", *codegenScope)