fix: update test operation generator to find any operation in the service closure (#3922)

## Motivation and Context
When using operations nested under resources, the current search will
raise "NoSuchElementException". This fixes the search to include all
operations within the service closure

## Description
The error is visible in client generates like this:
```
Projection rust-client failed: java.util.NoSuchElementException: Collection contains no element matching the predicate.
java.util.NoSuchElementException: Collection contains no element matching the predicate.
        at software.amazon.smithy.rustsdk.endpoints.OperationInputTestGeneratorKt.operationId(OperationInputTestGenerator.kt:242)
        at software.amazon.smithy.rustsdk.endpoints.OperationInputTestGenerator$operationInvocation$1.invoke(OperationInputTestGenerator.kt:183)
        at software.amazon.smithy.rustsdk.endpoints.OperationInputTestGenerator$operationInvocation$1.invoke(OperationInputTestGenerator.kt:180)
```

## Testing
`gradle :aws:sdk-codegen:check`
## Checklist


- [ ] 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.
- [ ] 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.

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