Remove startAsync() to prevent void methods from hanging forever

[kosmakdm]

Issue #, if available: #1598

Description of changes:

Root Cause

The springboot4 module unconditionally calls httpServletRequest.startAsync() before delegating to AwsSpringHttpProcessingUtils.processRequest(). This call present in both SpringDelegatingLambdaContainerHandler.handleRequest() and AwsSpringWebCustomRuntimeEventLoop.eventLoop(), it sets the request's dispatcher type to DispatcherType.ASYNC.

After ServerlessMVC.service() processes the request and the servlet/filter chain completes, there is a safety-net flush that ensures the response is always committed:

if (!response.isCommitted() && request.getDispatcherType() != DispatcherType.ASYNC) {
    response.flushBuffer();
}

Because startAsync() forces the dispatcher type to ASYNC, this safety net is skipped. For controllers that return a response body, this doesn't matter - Spring's HttpMessageConverter flushes the output stream during body serialization, which triggers AwsHttpServletResponse.flushBuffer() -> writersCountDownLatch.countDown(). But for void-returning controllers (e.g. a POST that creates a resource and returns nothing), no message converter runs, no flush ever happens, countDown() is never called, and latch.await(15, TimeUnit.MINUTES) blocks indefinitely.

The springboot3 module does NOT call startAsync(), so the safety-net flush always executes and the latch always counts down - even for void-returning controllers. This is why the same controller code works with Spring Boot 3 but hangs with Spring Boot 4.

Fix

Remove the unconditional httpServletRequest.startAsync() calls from both SpringDelegatingLambdaContainerHandler and AwsSpringWebCustomRuntimeEventLoop, aligning the springboot4 behavior with springboot3.

Changes

• SpringDelegatingLambdaContainerHandler.java: Removed httpServletRequest.startAsync() call
• AwsSpringWebCustomRuntimeEventLoop.java: Removed httpServletRequest.startAsync() call
• SpringDelegatingLambdaContainerHandlerTests.java: Added voidPost_returns200 test that verifies void-returning POST controllers complete successfully
• ServletApplication.java: Added voidPost() endpoint for the new test

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