Add idle body write detection to Netty HTTP client

[zoewangg]

Motivation and Context

When using a request body publisher that is slow or unable to produce data, the connection may stay open for a long time after headers are sent. The existing WriteTimeoutHandler doesn't detect this because no write() operation is scheduled for it to time out on.

Modifications

• Added WriteIdleTimeoutHandler that listens for WRITER_IDLE events from Netty's IdleStateHandler and proactively closes the connection with a descriptive error message
• Added IdleStateHandler + WriteIdleTimeoutHandler to the channel pipeline in NettyRequestExecutor.writeRequest(), placed after the SSL handler (before HttpStreamsClientHandler) to avoid TLS record writes resetting the idle timer
• Updated HandlerRemovingChannelPoolListener to clean up the new handlers on channel release

Testing

• WriteIdleTimeoutHandlerTest — unit tests for the handler: verifies idle event fires exception and closes channel, non-writer events are ignored, and duplicate events don't fire twice
• WriteIdleTimeoutTest — integration test with a TLS server and a stalled body publisher that never produces data, verifying the idle timeout fires within the expected window
• HandlerRemovingChannelPoolListenerTest — updated to verify new handlers are cleaned up on channel release

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have read the CONTRIBUTING document
• Local run of mvn clean install -pl :netty-nio-client succeeds
• My code follows the code style of this project
• I have added tests to cover my changes
• All new and existing tests passed
• I have added a changelog entry

License

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

[zoewangg]

We have to give it a dedicated name because we use IdleStateHandler elsewhere and removing the handler based on class type would cause those handlers to be removed prematurely