Call fallback exception handlers with the right exception

The issue presented in #26246 showed a deeper underlying problem. When
we fell back to the exception handler for an exceptions cause, we were
calling that handler with the outer raised exception. This breaks the
calling code's expectations, especially if the exception has methods on
it behond those from `StandardError`.

activesupport/lib/active_support/rescuable.rb

@@ -85,14 +85,16 @@ module ActiveSupport
       #
       # Returns the exception if it was handled and +nil+ if it was not.
       def rescue_with_handler(exception, object: self)
-        if handler = handler_for_rescue(exception, object: object)
+        handler, exception = handler_for_rescue(exception, object: object)
+        if handler
           handler.call exception
           exception
         end
       end
 
       def handler_for_rescue(exception, object: self) #:nodoc:
-        case rescuer = find_rescue_handler(exception)
+        rescuer, exception = find_rescue_handler(exception)
+        result = case rescuer
         when Symbol
           method = object.method(rescuer)
           if method.arity == 0
@@ -107,6 +109,7 @@ module ActiveSupport
             -> e { object.instance_exec(e, &rescuer) }
           end
         end
+        [result, exception]
       end
 
       private
@@ -121,7 +124,11 @@ module ActiveSupport
               end
             end
 
-            handler || find_rescue_handler(exception.cause)
+            if handler
+              [handler, exception]
+            else
+              find_rescue_handler(exception.cause)
+            end
           end
         end
 

activesupport/test/rescuable_test.rb

@@ -137,6 +137,6 @@ class RescuableTest < ActiveSupport::TestCase
 
   def test_rescue_falls_back_to_exception_cause
     @stargate.dispatch :fall_back_to_cause
-    assert_equal "unhandled RuntimeError with a handleable cause", @stargate.result
+    assert_equal "dex", @stargate.result
   end
 end