Fix ApiCallDuration so it measures the whole API call

Motivation and Context

CoreMetric.API_CALL_DURATION understated latency, and did so differently on the sync and async clients.

The measurement lived in a request pipeline stage. The pipeline's input is the already-marshalled request, so the window
opened after marshalling had finished — contradicting the formula in the metric's own javadoc, which lists a
MARSHALLING_DURATION term.

The async client was worse. AsyncApiCallMetricCollectionStage was nested one builder deeper than its sync counterpart,
so the window also excluded the entire request-mutation chain: endpoint resolution, auth scheme resolution, request compression, checksums, header and query merging and user agent.

The two clients also disagreed on the afterExecution interceptors: inside the window on async, outside on sync.

Modifications

• The measurement for ApiCallDuration is moved out of the pipeline entirely, up into the client handlers alongside API_CALL_SUCCESSFUL — which was already scoped to the whole call, so the two metrics had been reporting on different windows from the same collector. No pipeline arrangement can enclose marshalling, so this was the only available position.
• afterExecution interceptors are now inside the window on both clients: an interceptor running as part of the call is part of the call.
• SERVICE_ENDPOINT collection moved into the handlers' doExecute, the first point after the pipeline that still holds the ExecutionContext.
• Both handlers skip the clock reads when the collector is a NoOpMetricCollector, the default when no publisher is configured, so the no-metrics path does not regress.

Testing

New ApiCallDurationWindowTest (test/codegen-generated-classes-test) injects a 300 ms delay into one phase at a time and asserts it appears in ApiCallDuration, for both clients.

Types of changes

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

Checklist

• I have read the CONTRIBUTING document
• Local run of mvn install succeeds
• My code follows the code style of this project
• My change requires a change to the Javadoc documentation
• I have updated the Javadoc documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed
• I have added a changelog entry. Adding a new entry must be accomplished by running the scripts/new-change script and following the instructions. Commit the new file created by the script in .changes/next-release with your changes.

License

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

One internal-API behaviour change: code driving AmazonSyncHttpClient or AmazonAsyncHttpClient directly, bypassing the
client handlers, no longer receives ApiCallDuration or SERVICE_ENDPOINT.

Testing

New ApiCallDurationWindowTest (test/codegen-generated-classes-test) injects a 300 ms delay into one phase at a time
and asserts it appears in ApiCallDuration, for both clients. This is deliberately stronger than asserting the
additivity formula over real timings: the phases at issue cost microseconds against a millisecond-scale call, so an
inequality passes whether or not they are counted — an additivity-only check would not have caught the sync defect.

Verified against the unpatched SDK: 5 of the 7 cases fail, and the 2 that pass are exactly the two that were already
correct (sync endpoint resolution, async afterExecution).

Also added: ApiCallDurationAssertions, applied from CoreMetricsTest and BaseAsyncCoreMetricsTest, asserting
ApiCallDuration encloses each component metric on success, error and retry paths.