remote server: Do not spawn server when proxy reconnects (#18864)

This ensures that we only ever reconnect to a running server and not
spawn a new server with no state.

This avoids the problem of the server process crashing, `proxy`
reconnecting, starting a new server, and the user getting errors like
"unknown buffer id: ...".

Release Notes:

- N/A

---------

Co-authored-by: Bennet Bo Fenner <bennet@zed.dev>

crates/remote/src/proxy.rs

@@ -0,0 +1,25 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum ProxyLaunchError {
+    #[error("Attempted reconnect, but server not running.")]
+    ServerNotRunning,
+}
+
+impl ProxyLaunchError {
+    pub fn to_exit_code(&self) -> i32 {
+        match self {
+            // We're using 90 as the exit code, because 0-78 are often taken
+            // by shells and other conventions and >128 also has certain meanings
+            // in certain contexts.
+            Self::ServerNotRunning => 90,
+        }
+    }
+
+    pub fn from_exit_code(exit_code: i32) -> Option<Self> {
+        match exit_code {
+            90 => Some(Self::ServerNotRunning),
+            _ => None,
+        }
+    }
+}