MDL-78597 mod_lti: prevent manual instance use in edit form

This change ensures tool instances which are currently using
preconfigured tools (site or course level) are not able to be switched
back to use the manual 'Automatic, based on tool URL' option. That
option is reserved for legacy tool support of manual instances and
domain-matched site tools, but must not be selectable otherwise.

mod/lti/mod_form.php

@@ -156,7 +156,19 @@ class mod_lti_mod_form extends moodleform_mod {
             }
             $mform->addHelpButton('typeid', 'external_tool_type', 'lti');
 
-            foreach (lti_get_types_for_add_instance() as $id => $type) {
+            // To prevent the use of manually configured instances, existing instances which are using a preconfigured tool will not
+            // display the option "Automatic, based on tool URL" in the preconfigured tools select. This prevents switching from an
+            // instance configured using a preconfigured tool to an instance that is manually configured.
+            // Exceptions are made for:
+            // - Existing manually configured instances (i.e. no type set).
+            // - Instances using a tool type which isn't visible in the course preconfigured tools selector, such as when a
+            // site-level tool type is domain-matched.
+            $instancetypes = lti_get_types_for_add_instance();
+            if (!empty($this->current->typeid) && array_key_exists($this->current->typeid, $instancetypes)) {
+                unset($instancetypes[0]);
+            }
+
+            foreach ($instancetypes as $id => $type) {
                 if (!empty($type->toolproxyid)) {
                     $toolproxy[] = $type->id;
                     $attributes = array('globalTool' => 1, 'toolproxy' => 1);

mod/lti/tests/behat/addtool.feature

@@ -56,3 +56,5 @@ Feature: Add tools
     Then the field "Preconfigured tool" matches value "Course tool 1"
     And the "Select content" "button" should be disabled
     And the "Tool URL" "field" should be disabled
+    And I click on "Preconfigured tool" "select"
+    And I should not see "Automatic, based on tool URL"