wasi-http: dropping incoming-response silently closes request connection

[dicej]

While implementing a test for outbound request body streaming (#7412), I discovered that dropping the incoming-response object would cause the request connection to be closed, even if I still had open handles to the request body and stream.

For example, if I insert drop(response); to https://github.com/dicej/wasmtime/blob/1782a165754a2dd62f45cbc670603cc85477323b/crates/test-programs/src/bin/api_proxy_streaming.rs#L129, the test will fail with no (or few) bytes copied. The behavior I expected was that the test would pass (i.e. all bytes would be copied) regardless of whether the response handle was dropped or not.

[pchickey]

The worker task that manages the connection lives here: https://github.com/bytecodealliance/wasmtime/blob/main/crates/wasi-http/src/types.rs#L264 It looks like we need to hold an Arc to it both there and in the HostIncomingBody.