fix: add GUARDED_BY annotations to readiness Gate fields

Add required mutex guard annotations for the lint analyzer.

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

Author: ivanauth

pkg/middleware/readiness/readiness.go

@@ -47,9 +47,9 @@ type Gate struct {
 	sfGroup singleflight.Group
 
 	mu            sync.RWMutex
-	cachedReady   bool
-	cachedMessage string
-	cacheTime     time.Time
+	cachedReady   bool      // GUARDED_BY(mu)
+	cachedMessage string    // GUARDED_BY(mu)
+	cacheTime     time.Time // GUARDED_BY(mu)
 }
 
 // NewGate creates a new readiness gate with the given checker.