app-server: move transport into dedicated crate (#20545)

## Why

`codex-app-server` currently owns both request-processing code and
transport implementation details. Splitting the transport layer into its
own crate makes that boundary explicit, reduces the amount of
transport-specific dependency surface carried by `codex-app-server`, and
gives future transport work a narrower place to evolve.

## What changed

- Added `codex-app-server-transport` and moved the existing transport
tree into it, including stdio, unix socket, websocket, remote-control
transport, and websocket auth.
- Moved shared transport-facing message types into the new crate so both
the transport implementation and `codex-app-server` use the same
definitions.
- Kept processor-facing connection state and outbound routing in
`codex-app-server`, with the routing tests moved next to that local
wrapper.
- Updated workspace metadata, Bazel crate metadata, and
`codex-app-server` dependencies for the new crate boundary.

## Validation

- `cargo metadata --locked --no-deps`
- `git diff --check`
- Attempted `cargo test -p codex-app-server-transport`, `cargo test -p
codex-app-server`, `just fix -p codex-app-server-transport`, and `just
fix -p codex-app-server`; all were blocked before compilation by the
existing `packageproxy` resolution failure for locked `rustls-webpki =
0.103.13`.
- Attempted Bazel build / lockfile validation; those were blocked by
external fetch failures against BuildBuddy / GitHub while resolving
`v8`.

codex-rs/app-server-transport/src/outgoing_message.rs

@@ -0,0 +1,58 @@
+use std::fmt;
+
+use codex_app_server_protocol::JSONRPCErrorError;
+use codex_app_server_protocol::RequestId;
+use codex_app_server_protocol::Result;
+use codex_app_server_protocol::ServerNotification;
+use codex_app_server_protocol::ServerRequest;
+use serde::Serialize;
+use tokio::sync::oneshot;
+
+/// Stable identifier for a transport connection.
+#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+pub struct ConnectionId(pub u64);
+
+impl fmt::Display for ConnectionId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
+/// Outgoing message from the server to the client.
+#[derive(Debug, Clone, Serialize)]
+#[serde(untagged)]
+pub enum OutgoingMessage {
+    Request(ServerRequest),
+    /// AppServerNotification is specific to the case where this is run as an
+    /// "app server" as opposed to an MCP server.
+    AppServerNotification(ServerNotification),
+    Response(OutgoingResponse),
+    Error(OutgoingError),
+}
+
+#[derive(Debug, Clone, PartialEq, Serialize)]
+pub struct OutgoingResponse {
+    pub id: RequestId,
+    pub result: Result,
+}
+
+#[derive(Debug, Clone, PartialEq, Serialize)]
+pub struct OutgoingError {
+    pub error: JSONRPCErrorError,
+    pub id: RequestId,
+}
+
+#[derive(Debug)]
+pub struct QueuedOutgoingMessage {
+    pub message: OutgoingMessage,
+    pub write_complete_tx: Option<oneshot::Sender<()>>,
+}
+
+impl QueuedOutgoingMessage {
+    pub fn new(message: OutgoingMessage) -> Self {
+        Self {
+            message,
+            write_complete_tx: None,
+        }
+    }
+}