Restore SmokeTestsDecoratorTest

[ysaito1001]

Motivation and Context

Restores SmokeTestsDecoratorTest that was temporarily removed in #3808.

Description

The said test was temporarily removed because SmokeTestsDecorator is included in the predefined decorators and pulls-in the aws-config crate for code generation. The issue was conflicts between the runtime crates used by aws-config (located in the aws/sdk/build directory) and those brought-in by that predefined decorators used by awsSdkIntegrationTest (located in the rust-runtime and aws/rust-runtime directories). This PR addresses these conflicts and restores the test functionality.

Given the challenges, the code changes are centered around the following ideas:

• Focus solely on testing the core class, SmokeTestsInstantiator. By doing this, we avoid running SmokeTestsDecorator, which would otherwise pull in the aws-config crate.
• Initializing a config builder in smoke tests needs to be parameterized depending on the environment; in production we use aws_config::load_defaults and in testing we avoid including aws-config by using a default-constructed config builder, which is sufficient for compile-only tests.
• The generated smoke tests in SmokeTestsDecoratorTest require minimal runtime crates for compilation. We need the CodegenVisitor but with a different set of codegen decorators. Prior to this PR, awsSdkIntegrationTest used RustClientCodegenPlugin that in turn loaded predefined decorators on the classpath, which included SmokeTestsDecorator. In this PR, we work around this by defining a minimal set of codegen decorators and making them pluggable through awsSdkIntegrationTest.

Testing

Tests in CI

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[github-actions]

A new generated diff is ready to view.

• No codegen difference in the AWS SDK
• No codegen difference in the Client Test
• No codegen difference in the Server Test
• No codegen difference in the Server Test Python
• No codegen difference in the Server Test Typescript

A new doc preview is ready to view.

[github-actions]

A new generated diff is ready to view.

• No codegen difference in the AWS SDK
• No codegen difference in the Client Test
• No codegen difference in the Server Test
• No codegen difference in the Server Test Python
• No codegen difference in the Server Test Typescript

A new doc preview is ready to view.

[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• No codegen difference in the Client Test
• No codegen difference in the Server Test
• No codegen difference in the Server Test Python
• No codegen difference in the Server Test Typescript

A new doc preview is ready to view.