Close a UDP proxy backend that becomes active after eviction

[devops-thiago]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

Fixes #2015.

UDPProxyFrontend keeps at most 256 backends in an LRU cache and closes whichever one is evicted. UDPProxyBackend.close() returned early when the backend channel had not finished binding:

func close() {
    guard let channel = state.channel else {
        self.log?.warning("backend - close on inactive channel")
        return
    }
    _ = channel.close()
}

state.channel is nil from construction until channelActive runs, which happens after bind completes. A backend evicted inside that window is never closed. channelActive then adopts the channel, and because the ProxyContext has already been dropped from the cache, no reference remains that could close it. The socket stays open for the lifetime of the process.

The existing warning shows the case was anticipated, but the early return leaves the socket open rather than deferring the close.

This change records the close request in the backend state and has channelActive honour it, closing the channel instead of adopting it and discarding any queued payloads. Both paths run on the event loop the backend is bootstrapped on, since DatagramBootstrap(group: context.eventLoop) and NIOLoopBound(proxy, eventLoop: context.eventLoop) bind them to that loop, so the flag needs no further synchronisation.

UDPProxyBackend changes from file-private to internal so the test can drive the ordering directly. Nothing outside the module references it.

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

testBackendClosedBeforeChannelActiveClosesTheChannel calls close() before the channel becomes active, then activates it through an EmbeddedChannel and asserts the channel is no longer active. The test fails against the current code and passes with this change. testUDPForwarder and testLRUCache continue to pass.