[multi-ail-detector] allow detector to run independently of Border Routing (#12078)

This commit updates the Multi-AIL Detection feature to operate
independently of the Border Routing Manager. This fundamental change
allows the detector to be enabled/disabled on its own, rather than
being tied to the Border Routing Manager's state.

This change also moves the Multi-AIL detection API into a separate
`openthread/multi_ail_detection.h` header and introduces new APIs to
control the detector independently. Corresponding CLI commands are
also added.

The `test-505-multi-ail-detection.py` is also updated to validate
this new independent behavior. In particular that a device that is
not enabled to act as a BR can independently run multi-AIL detection
and determine whether, if it becomes a BR, it will cause multi-AIL
issues.

tests/unit/test_routing_manager.cpp

@@ -1416,6 +1416,18 @@ void InitTest(bool aEnablBorderRouting = false, bool aAfterReset = false)
 
     SuccessOrQuit(otIp6SetEnabled(sInstance, true));
     SuccessOrQuit(otThreadSetEnabled(sInstance, true));
+
+#if OPENTHREAD_CONFIG_BORDER_ROUTING_MULTI_AIL_DETECTION_ENABLE
+    // We explicitly disable the multi-AIL detector to prevent
+    // interference with the tests. The detector enables
+    // `RxRaTracker` and keeps it enabled even when Border Routing is
+    // disabled, which keeps prefix entries in the heap and can
+    // invalidate the heap allocation checks. It may also mess up the
+    // expected timing of RS (Router Solicitation) messages (starting
+    // the `RxRaTracker` early).
+    otBorderRoutingSetMultiAilDetectionEnabled(sInstance, false);
+#endif
+
     SuccessOrQuit(otBorderRoutingSetEnabled(sInstance, aEnablBorderRouting));
 
     // Reset all test flags