Fix (and test) for a deadlock that can happen while waiting for protocol info (#12633)

# Description

The local socket PR introduced a `Waitable` type, which could either
hold a value or be waited on until a value is available. Unlike a
channel, it would always return that value once set.

However, one issue with this design was that there was no way to detect
whether a value would ever be written. This splits the writer into a
different type `WaitableMut`, so that when it is dropped, waiting
threads can fail (because they'll never get a value).

# Tests + Formatting

A test has been added to `stress_internals` to make sure this fails in
the right way.

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

crates/nu_plugin_stress_internals/src/main.rs

@@ -11,6 +11,7 @@ use serde_json::{json, Value};
 struct Options {
     refuse_local_socket: bool,
     advertise_local_socket: bool,
+    exit_before_hello: bool,
     exit_early: bool,
     wrong_version: bool,
     local_socket_path: Option<String>,
@@ -28,6 +29,7 @@ pub fn main() -> Result<(), Box<dyn Error>> {
     let mut opts = Options {
         refuse_local_socket: has_env("STRESS_REFUSE_LOCAL_SOCKET"),
         advertise_local_socket: has_env("STRESS_ADVERTISE_LOCAL_SOCKET"),
+        exit_before_hello: has_env("STRESS_EXIT_BEFORE_HELLO"),
         exit_early: has_env("STRESS_EXIT_EARLY"),
         wrong_version: has_env("STRESS_WRONG_VERSION"),
         local_socket_path: None,
@@ -75,6 +77,11 @@ pub fn main() -> Result<(), Box<dyn Error>> {
         output.flush()?;
     }
 
+    // Test exiting without `Hello`
+    if opts.exit_before_hello {
+        std::process::exit(1)
+    }
+
     // Send `Hello` message
     write(
         &mut output,