feat(codegen): generate sync and async client instantiation in ServiceWarmUpProvider.warmUp()

[joviegas]

Motivation and Context

CRaC auto-priming runs each service's warm-up code before a checkpoint so the first call after restore is fast. PR #7085 generates a <Service>WarmUpProvider class per service, but its warmUp() method is empty.

This PR fills warmUp() with client creation: build and close a sync and an async client that use an in-memory HTTP client instead of the network. This makes the JVM compile the client build and config code during warm-up. Calling an actual operation is a later PR.
Note that the operation call will be added in upcoming PRs

Modifications

• Add CannedResponseAsyncHttpClient in sdk-core. It is the async version of the existing CannedResponseHttpClient. It returns a fixed status code and body using a SimplePublisher and does no network I/O.
• Change WarmUpProviderSpec so warmUp() creates and closes, using try-with-resources:
  • a sync client, only when the service generates one (!skipSyncClientGeneration);
  • an async client, always.
    Each client uses dummy credentials, Region.US_EAST_1, and a local endpointOverride. The try blocks are empty; no operation is called.
• Add a per-protocol CANNED_RESPONSE byte constant: {} for json/rest-json/cbor, <Response/> for query/ec2/rest-xml, and an empty CBOR map for smithy-rpc-v2-cbor.
• Add tests: CannedResponseAsyncHttpClientTest, and WarmUpProviderSpecTest cases per protocol and for the sync-skipped (async-only) case.

Verified: codegen build passes checkstyle and 587 tests; sdk-core passes spotbugs; sqs, transcribestreaming (async-only), and codegen-generated-classes-test regenerate and compile.

Testing

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

License

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