fix(exams): remove isolating to allow nested question blocks

Root cause: questionBlock/groupBlock/sectionBlock had `isolating: true`,
which prevents ProseMirror operations (wrapIn, insertContentAt) from
inserting nodes inside these blocks. When users selected text inside a
composite question and clicked "填空/简答" to create a sub-question,
the new questionBlock was inserted OUTSIDE the composite block instead
of inside it, so buildQuestion couldn't detect it as a subQuestion.

Fix: remove `isolating: true` from all three block extensions. The
`defining: true` property is sufficient to maintain node boundaries
during normal editing operations.

This allows wrapInQuestion to work correctly inside composite questions,
creating properly nested sub-question blocks that are detected by
buildQuestion's recursive parsing.

src/modules/exams/editor/extensions/group-block.tsx

@@ -80,7 +80,6 @@ export const GroupBlock = Node.create({
   group: "block",
   content: "block+",
   defining: true,
-  isolating: true,
   addAttributes() {
     return {
       title: { default: "" },

src/modules/exams/editor/extensions/question-block.tsx

@@ -64,7 +64,6 @@ export const QuestionBlock = Node.create({
   group: "block",
   content: "block+",
   defining: true,
-  isolating: true,
   addAttributes() {
     return {
       questionId: { default: "" },

src/modules/exams/editor/extensions/section-block.tsx

@@ -86,7 +86,6 @@ export const SectionBlock = Node.create({
   group: "block",
   content: "block+",
   defining: true,
-  isolating: true,
   addAttributes() {
     return {
       title: { default: "" },