Fix ACP connection and thread leak (#35670)

When you switched away from an ACP thread, the `AcpThreadView` entity
(and thus thread, and subprocess) was leaked. This happened because we
were using `cx.processor` for the `list` state callback, which uses a
strong reference.

This PR changes the callback so that it holds a weak reference, and adds
some tests and assertions at various levels to make sure we don't
reintroduce the leak in the future.

Release Notes:

- N/A

Cargo.lock

@@ -137,9 +137,9 @@ dependencies = [
 
 [[package]]
 name = "agent-client-protocol"
-version = "0.0.18"
+version = "0.0.20"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f8e4c1dccb35e69d32566f0d11948d902f9942fc3f038821816c1150cf5925f4"
+checksum = "12dbfec3d27680337ed9d3064eecafe97acf0b0f190148bb4e29d96707c9e403"
 dependencies = [
  "anyhow",
  "futures 0.3.31",