fix: Add Go 1.25+ build tag for WaitGroup.Go spawnerlabel tests

Split sync.WaitGroup.Go tests into separate file with go1.25 build tag
to maintain compatibility with both Go 1.24 and 1.25.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: mpyw

testdata/src/spawnerlabel/waitgroup_go125.go

@@ -0,0 +1,37 @@
+//go:build go1.25
+
+package spawnerlabel
+
+import (
+	"fmt"
+	"sync"
+)
+
+// [BAD]: Missing label - calls sync.WaitGroup.Go with func arg
+func missingLabelWaitgroup() { // want `function "missingLabelWaitgroup" should have //goroutinectx:spawner directive \(calls sync\.WaitGroup\.Go with func argument\)`
+	var wg sync.WaitGroup
+	wg.Go(func() {
+		fmt.Println("work")
+	})
+	wg.Wait()
+}
+
+// [GOOD]: Traditional WaitGroup pattern (Add/Done) - not spawn method
+func goodTraditionalWaitgroup() {
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		fmt.Println("work")
+	}()
+	wg.Wait()
+}
+
+// [BAD]: Multiple spawn types including WaitGroup.Go
+func multipleSpawnTypesWithWaitgroup() { // want `function "multipleSpawnTypesWithWaitgroup" should have //goroutinectx:spawner directive \(calls sync\.WaitGroup\.Go with func argument\)`
+	var wg sync.WaitGroup
+	wg.Go(func() {
+		fmt.Println("work")
+	})
+	wg.Wait()
+}