Ensure a lost connection upon COMMIT fails "cleanly"

There's no point attempting to rollback in this case; we can instead
just invalidate the now-lost transaction.

Similarly, even though we can't immediately reconnect for a connection
failure mid-transaction, we can and should drop and prior verification
of the connection: the next out-of-transaction query attempt needs to
fix it, not assume it's working.

activerecord/lib/active_record/connection_adapters/abstract/transaction.rb

@@ -495,6 +495,9 @@ def within_new_transaction(isolation: nil, joinable: true)
 
                 begin
                   commit_transaction
+                rescue ActiveRecord::ConnectionFailed
+                  transaction.state.invalidate! unless transaction.state.completed?
+                  raise
                 rescue Exception
                   rollback_transaction(transaction) unless transaction.state.completed?
                   raise

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -941,6 +941,12 @@ def with_raw_connection(allow_retry: false, uses_transaction: true)
                 end
               end
 
+              if retryable_connection_error?(translated_exception)
+                # There's a problem with the connection, but we couldn't attempt to
+                # reconnect. The connection is going to stay broken, so un-verify it.
+                @verified = false
+              end
+
               raise translated_exception
             ensure
               dirty_current_transaction if uses_transaction