fix: vendor deps and fix CI go-version, timeout, GOTOOLCHAIN (HAN-414)

- Run go mod vendor to eliminate proxy.golang.org network dependency
- Set GOFLAGS=-mod=vendor at job env level so all go commands use vendor/
- Fix go-version from 1.23 to 1.24.0 to match go.mod
- Move GOTOOLCHAIN=local to job-level env (not per-step inline)
- Set cache: false on setup-go (cache is irrelevant with vendoring)
- Add timeout-minutes: 15 so failures are caught quickly instead of 3h

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: multica-agent <github@multica.ai>

vendor/golang.org/x/net/ipv6/payload.go

@@ -0,0 +1,23 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ipv6
+
+import (
+	"net"
+
+	"golang.org/x/net/internal/socket"
+)
+
+// BUG(mikio): On Windows, the ControlMessage for ReadFrom and WriteTo
+// methods of PacketConn is not implemented.
+
+// A payloadHandler represents the IPv6 datagram payload handler.
+type payloadHandler struct {
+	net.PacketConn
+	*socket.Conn
+	rawOpt
+}
+
+func (c *payloadHandler) ok() bool { return c != nil && c.PacketConn != nil && c.Conn != nil }