chore: bump x to v0.15.0 (hopGroup, failCodes fix)

Add e2e regression test for http.failCodes selector bug —
FIFO selector + failCodes=429 on first node proves
node-429 is excluded after first 429 response and all
subsequent requests go to node-good.

tests/e2e/scripts/http_status_backend.py

@@ -0,0 +1,35 @@
+"""HTTP server that always returns a fixed status code."""
+import sys
+from http.server import BaseHTTPRequestHandler, HTTPServer
+
+
+STATUS = int(sys.argv[1]) if len(sys.argv) > 1 else 429
+PORT = int(sys.argv[2]) if len(sys.argv) > 2 else 5680
+BODY = f"status-{STATUS}".encode()
+
+
+class Handler(BaseHTTPRequestHandler):
+    def do_GET(self):
+        self.send_response(STATUS)
+        self.send_header("Content-Length", str(len(BODY)))
+        self.end_headers()
+        self.wfile.write(BODY)
+
+    def do_POST(self):
+        self.send_response(STATUS)
+        self.send_header("Content-Length", str(len(BODY)))
+        self.end_headers()
+        self.wfile.write(BODY)
+
+    # HTTP proxy CONNECT method
+    def do_CONNECT(self):
+        self.send_response(STATUS)
+        self.send_header("Content-Length", str(len(BODY)))
+        self.end_headers()
+        self.wfile.write(BODY)
+
+    def log_message(self, format, *args):
+        return
+
+
+HTTPServer(("0.0.0.0", PORT), Handler).serve_forever()