fix: update test operation generator to find any operation in the service closure

[mcmasn-amzn]

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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