Fix 6 issues surfaced by adversarial agent team (round 35)

Concurrency/lifecycle (P1, highest-value):
- singleton_resources.go: RegisterBackgroundTask now REPLACES a stopped
  task instead of reusing it. BackgroundTask.Start is sync.Once-guarded,
  so a task stopped on instance close and re-registered (Traefik
  close+recreate for the same provider) was permanently dead: IsTaskRunning
  returned false (started && !stopped) so callers retried a start that
  was a no-op. Metadata refresh, token cleanup, and health checks now
  resume across a recreate cycle.
- error_recovery.go: the shared graceful-degradation health task was
  bound to the FIRST instance's performHealthChecks (later instances'
  checks never ran) and any instance's Close stopped it for all. Added a
  global live-instance registry; the shared task runs every instance's
  checks and is stopped only when the last instance closes (mirrors the
  singleton-token-cleanup gate).

Storage/cache:
- hybrid.go recordL2Error: Store(now) then immediately compared
  time.Since(t)<1s against its own write => ALWAYS fired, so a single
  transient L2 error flipped the whole cache into fallback (full L2
  bypass -> upstream stampede). Now enters fallback only on a PRIOR
  error within the window (true "multiple errors"). Updated two tests
  that had encoded the old single-error-fallback bug.
- redis_pool.go: ConnectionPool.Put checked closed but sent on
  p.connections without holding p.mu; Close closed it under p.mu.
  Window => 'send on closed channel' panic at shutdown. Put now holds
  p.mu (the lock only Close used).

HTTP/transport:
- jwk.go fetchJWKS: added an explicit >1 MiB guard (was silent partial
  read, could yield a truncated keyset and spurious missing-kid).
- http_client_pool.go configKey: now includes every transport-behavior
  field it actually consumes (compression, HTTP2, keep-alives, timeouts,
  buffer sizes, dial) so behaviorally different configs no longer share
  one transport built with the first config's settings.

Each fix has a regression test. Verified: full source-order suite green,
golangci-lint 0 issues, 3x full -race -count=1 -shuffle=on ./... all exit 0.

singleton_resources.go

@@ -123,16 +123,23 @@ func (rm *ResourceManager) RegisterBackgroundTask(name string, interval time.Dur
 	rm.tasksMu.Lock()
 	defer rm.tasksMu.Unlock()
 
-	// Check if task already exists
-	if _, exists := rm.tasks[name]; exists {
-		if rm.logger != nil {
-			rm.logger.Debugf("Background task %s already registered", name)
+	// If a task with this name already exists and is still running, keep it
+	// (idempotent re-registration from another middleware instance).
+	if existing, exists := rm.tasks[name]; exists {
+		if atomic.LoadInt32(&existing.stopped) == 0 {
+			if rm.logger != nil {
+				rm.logger.Debugf("Background task %s already registered", name)
+			}
+			return nil
 		}
-		// Return existing task without error for idempotency
-		return nil
+		// The existing task was stopped. Reusing the same BackgroundTask
+		// object would leave it permanently dead: BackgroundTask.Start is
+		// guarded by sync.Once, so once stopped it can never restart, yet
+		// IsTaskRunning returns false and callers will call Start again.
+		// Replace it with a fresh task so re-registration after a
+		// close+recreate cycle (same provider URL) actually resumes it.
 	}
 
-	// Create new task with WaitGroup for proper cleanup
 	task := NewBackgroundTask(name, interval, taskFunc, rm.logger, &rm.wg)
 	rm.tasks[name] = task