Fix singleton ResourceManager data race (unlocked read vs test reset)

GetResourceManager returned the process-global via an unprotected read
(singleton_resources.go:50), while resetResourceManagerForTesting and
ResetResourceManager write it under resourceManagerMutex. A live background
task (graceful-degradation health check started by one test) reading the
singleton while a shuffled test resets it was a data race (found via
-race -shuffle: Write at singleton_resources_test.go:941/942 vs Read at
autocleanup.go:554 / error_recovery.go:910).

Fix: hold resourceManagerMutex around the read, matching the sibling
GetGlobalTaskRegistry (autocleanup.go:414). Added a deterministic
concurrent regression test (GetResourceManager vs reset under -race).

Verified: 4x full -race -count=1 -shuffle=on ./... all exit 0, 0 races,
0 fails; source-order suite green; lint clean.

singleton_resources.go

@@ -36,6 +36,9 @@ type ResourceManager struct {
 
 // GetResourceManager returns the global singleton ResourceManager instance
 func GetResourceManager() *ResourceManager {
+	resourceManagerMutex.Lock()
+	defer resourceManagerMutex.Unlock()
+
 	resourceManagerOnce.Do(func() {
 		globalResourceManager = &ResourceManager{
 			httpClients:  make(map[string]*http.Client),

singleton_resources_race_test.go

@@ -0,0 +1,41 @@
+package traefikoidc
+
+import (
+	"sync"
+	"testing"
+)
+
+// TestConcurrentGetResourceManagerVsReset stresses GetResourceManager against
+// resetResourceManagerForTesting under -race. GetResourceManager returns the
+// process-global via an unprotected read unless the caller holds
+// resourceManagerMutex (regression: a live background task reading the
+// singleton while a shuffled test resets it was a data race). Run with -race;
+// this fails (race) before the mutex fix and passes after.
+func TestConcurrentGetResourceManagerVsReset(t *testing.T) {
+	var wg sync.WaitGroup
+
+	// Readers constantly re-fetch the global, as a background task would.
+	for g := 0; g < 8; g++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for i := 0; i < 2000; i++ {
+				if rm := GetResourceManager(); rm == nil {
+					t.Log("GetResourceManager returned nil")
+				}
+			}
+		}()
+	}
+
+	// Resetter repeatedly pins/unpins the singleton with the test helper,
+	// mirroring resetResourceManagerForTesting's writes in a shuffled suite.
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		for i := 0; i < 200; i++ {
+			resetResourceManagerForTesting()
+		}
+	}()
+
+	wg.Wait()
+}