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/github.com/miekg/dns/fuzz.go

@@ -0,0 +1,33 @@
+//go:build fuzz
+// +build fuzz
+
+package dns
+
+import "strings"
+
+func Fuzz(data []byte) int {
+	msg := new(Msg)
+
+	if err := msg.Unpack(data); err != nil {
+		return 0
+	}
+	if _, err := msg.Pack(); err != nil {
+		return 0
+	}
+
+	return 1
+}
+
+func FuzzNewRR(data []byte) int {
+	str := string(data)
+	// Do not fuzz lines that include the $INCLUDE keyword and hint the fuzzer
+	// at avoiding them.
+	// See GH#1025 for context.
+	if strings.Contains(strings.ToUpper(str), "$INCLUDE") {
+		return -1
+	}
+	if _, err := NewRR(str); err != nil {
+		return 0
+	}
+	return 1
+}