[V4] Update existing service tests to use async operations

[dscpinheiro]

Follow up for #4177, this PR updates the tests for services with hand-written code to use the Async operations. Most of the changes here are similar to Client.GetRestApi -> await Client.GetRestApiAsync but some important things to call out:

• I did not include S3 or DDB since those are the majority of tests (and there's parallel work on those services). I'll address them in their own PR later
• If the tests were not validating the custom code, I deleted the test project altogether. For example, the hand-written code for CloudFormation is about updating a stack (https://github.com/aws/aws-sdk-net/tree/main/sdk/src/Services/CloudFormation/Custom), but we were testing other operations such as ValidateTemplate instead (with a aws-vs-toolkit.s3.amazonaws.com URL...)
• If the test was broken and already ignored, I deleted it as well. For example, Bedrock had a test for event streaming but a) the model identifier is not supported anymore, and b) newer models require a different syntax.
• (This is the most controversial one) If the tests are not being run in our build system today, I got rid of them as well. There are some tests that need long-term credentials (i.e. an IAM user) and are marked with a specific category (which we ignore during releases). Note most of them aren't for custom code but I thought I might as well include them in the refactoring.

Testing

Dry-run: DRY_RUN-f2020ce8-4c7b-4f82-80aa-24b63d923a66

License

• I confirm that this pull request can be released under the Apache 2 license