Add exec-server stub server and protocol docs (#15089)

Stacked PR 1/3.

This is the initialize-only exec-server stub slice: binary/client
scaffolding and protocol docs, without exec/filesystem implementation.

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/exec-server/src/server/jsonrpc.rs

@@ -0,0 +1,53 @@
+use codex_app_server_protocol::JSONRPCError;
+use codex_app_server_protocol::JSONRPCErrorError;
+use codex_app_server_protocol::JSONRPCMessage;
+use codex_app_server_protocol::JSONRPCResponse;
+use codex_app_server_protocol::RequestId;
+use serde_json::Value;
+
+pub(crate) fn invalid_request(message: String) -> JSONRPCErrorError {
+    JSONRPCErrorError {
+        code: -32600,
+        data: None,
+        message,
+    }
+}
+
+pub(crate) fn invalid_params(message: String) -> JSONRPCErrorError {
+    JSONRPCErrorError {
+        code: -32602,
+        data: None,
+        message,
+    }
+}
+
+pub(crate) fn method_not_found(message: String) -> JSONRPCErrorError {
+    JSONRPCErrorError {
+        code: -32601,
+        data: None,
+        message,
+    }
+}
+
+pub(crate) fn response_message(
+    request_id: RequestId,
+    result: Result<Value, JSONRPCErrorError>,
+) -> JSONRPCMessage {
+    match result {
+        Ok(result) => JSONRPCMessage::Response(JSONRPCResponse {
+            id: request_id,
+            result,
+        }),
+        Err(error) => JSONRPCMessage::Error(JSONRPCError {
+            id: request_id,
+            error,
+        }),
+    }
+}
+
+pub(crate) fn invalid_request_message(reason: String) -> JSONRPCMessage {
+    JSONRPCMessage::Error(JSONRPCError {
+        id: RequestId::Integer(-1),
+        error: invalid_request(reason),
+    })
+}