debugger: Fix endless restarts when connecting to TCP adapters over SSH (cherry-pick #34328) (#34343)

Cherry-picked debugger: Fix endless restarts when connecting to TCP
adapters over SSH (#34328)

Closes #34323
Closes #34313

The previous PR #33932 introduced a way to "close" the
`pending_requests` buffer of the `TransportDelegate`, preventing any
more requests from being added. This prevents pending requests from
accumulating without ever being drained during the shutdown sequence;
without it, some of our tests hang at this point (due to using a
single-threaded executor).

The bug occurred because we were closing `pending_requests` whenever we
detected the server side of the transport shut down, and this closed
state stuck around and interfered with the retry logic for SSH+TCP
adapter connections.

This PR fixes the bug by only closing `pending_requests` on session
shutdown, and adds a regression test covering the SSH retry logic.

Release Notes:

- debugger: Fixed a bug causing SSH connections to some adapters
(Python, Go, JavaScript) to fail and restart endlessly.

Co-authored-by: Cole Miller <cole@zed.dev>

crates/dap/src/adapters.rs

@@ -442,10 +442,18 @@ impl DebugAdapter for FakeAdapter {
         _: Option<Vec<String>>,
         _: &mut AsyncApp,
     ) -> Result<DebugAdapterBinary> {
+        let connection = task_definition
+            .tcp_connection
+            .as_ref()
+            .map(|connection| TcpArguments {
+                host: connection.host(),
+                port: connection.port.unwrap_or(17),
+                timeout: connection.timeout,
+            });
         Ok(DebugAdapterBinary {
             command: Some("command".into()),
             arguments: vec![],
-            connection: None,
+            connection,
             envs: HashMap::default(),
             cwd: None,
             request_args: StartDebuggingRequestArguments {