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.

Cargo.lock

@@ -3109,6 +3109,7 @@ dependencies = [
  "context_server",
  "ctor",
  "dap",
+ "dap-types",
  "dap_adapters",
  "dashmap 6.1.0",
  "debugger_ui",