fix: producer is not closed at server side when batching is enabled

[BewareMyPower]

Fixes #360

[Copilot]

Pull Request Overview

This PR fixes issue #360 where the producer was not properly closed at the server side when batching is enabled. The fix ensures that when a batched producer is closed, the CloseProducer RPC call is properly sent to the server and the result is propagated back to the caller.

Key changes:

• Modified the close mechanism to properly handle RPC communication between batch processing loops
• Updated the close receiver to return the result of the CloseProducer RPC call instead of just a unit value
• Added test coverage to verify that producers are removed from the server's publisher list after closing

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/producer.rs	Modified batch processing to properly handle producer closure with RPC result propagation
src/lib.rs	Added test to verify producer removal from server-side publisher list
Cargo.toml	Added reqwest dependency for HTTP calls in tests

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