[exec-server] serve websocket listener via HTTP upgrade (#21963)

## Why

`codex exec-server` should keep the existing public `ws://IP:PORT` URL
shape while serving that websocket connection through an HTTP upgrade
path internally. That keeps the client-facing configuration simple and
allows the listener to work through intermediate HTTP-aware
infrastructure.

## What changed

- keep the emitted and configured exec-server URL as `ws://IP:PORT`
- serve that websocket endpoint through Axum HTTP upgrade handling on
`/`
- expose `GET /readyz` from the same listener for readiness checks
- route upgraded Axum websocket streams through the shared JSON-RPC
connection machinery
- initialize the rustls crypto provider before websocket client
connections
- preserve inbound binary websocket JSON-RPC parsing for compatibility
with the prior transport behavior

## Verification

- `cargo test -p codex-exec-server --test health --test process --test
websocket --test initialize --test exec_process`

codex-rs/exec-server/tests/common/exec_server.rs

@@ -142,6 +142,11 @@ impl ExecServerHarness {
         Ok(())
     }
 
+    pub(crate) async fn send_raw_binary(&mut self, bytes: Vec<u8>) -> anyhow::Result<()> {
+        self.websocket.send(Message::Binary(bytes.into())).await?;
+        Ok(())
+    }
+
     pub(crate) async fn next_event(&mut self) -> anyhow::Result<JSONRPCMessage> {
         self.next_event_with_timeout(EVENT_TIMEOUT).await
     }