fix(comment): dismiss owner/admin authz + atomic conditional delete + 404-only onError (#329 review)

Maintainer escalation decision (B) + reviewer findings on the ephemeral-
suggestion PR.

Authz (decision B): POST /comments/dismiss-suggestion now gates the destructive
branch on owner-OR-space-admin, mirroring POST /comments/delete exactly (same
SpaceCaslAction.Manage / SpaceCaslSubject.Settings, same owner short-circuit,
same ForbiddenException). A non-owner non-admin who tries to dismiss another's
childless suggestion gets Forbidden before the service runs. Apply stays on
canEdit (accepting an edit is the editor's semantics), unchanged.

F1 [blocking] — atomic conditional delete closes the hasChildren→delete race.
New repo `deleteCommentIfChildless(id)` runs a single
`DELETE FROM comments WHERE id=:id AND NOT EXISTS (SELECT 1 FROM comments child
WHERE child.parent_comment_id = comments.id)` (verified by compiling the Kysely
expression to SQL — the correlated subquery references the OUTER comments.id).
deleteEphemeralSuggestion strips the mark first, then the conditional delete: if
it removed the row → commentDeleted + outcome 'deleted'; if a reply raced in
(0 rows) → fall back to resolveComment (outcome 'resolved') so the discussion and
the new reply survive. No reply can be cascade-deleted anymore.

F2 [warning] — the apply/dismiss onError success-noop is narrowed from 404||400
to 404 ONLY. A 400 means the comment is ALIVE (apply's 400 = the thread was
resolved-not-applied), so it now shows a real error (surfacing the server
message) and KEEPS the comment in cache instead of a false "applied" + dropping a
live thread.

F3 [suggestion] — the 404-race client tests assert the success toast fired.

Tests: server — dismiss authz (owner ok / non-owner-non-admin Forbidden /
space-admin ok), the delete→resolve race (hasChildren=false but conditional
delete returns 0 → resolve, no commentDeleted), delete-path asserts switched to
deleteCommentIfChildless; client — apply-400 and dismiss-400 (kept in cache, red,
not success) + the toast assertions.

server tsc clean, comment+collaboration jest green; client tsc clean, comment
vitest 54 passed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/server/src/core/comment/comment.controller.spec.ts

@@ -127,7 +127,9 @@ describe('CommentController apply-suggestion authz', () => {
  * the delete/resolve + mark removal). These tests pin that boundary.
  */
 describe('CommentController dismiss-suggestion authz', () => {
-  function makeController() {
+  // isAdmin=false → ability.cannot(Manage, Settings) returns true (i.e. the user
+  // is NOT a space admin). Flip to true to model a space admin.
+  function makeController(isAdmin = false) {
     const commentService = {
       dismissSuggestion: jest.fn(async () => ({
         id: 'c-1',
@@ -136,7 +138,11 @@ describe('CommentController dismiss-suggestion authz', () => {
     };
     const commentRepo = { findById: jest.fn() };
     const pageRepo = { findById: jest.fn() };
-    const spaceAbility = {} as any;
+    const spaceAbility = {
+      createForUser: jest.fn(async () => ({
+        cannot: jest.fn(() => !isAdmin),
+      })),
+    } as any;
     const pageAccessService = {
       validateCanComment: jest.fn(async () => undefined),
       validateCanEdit: jest.fn(async () => undefined),
@@ -159,6 +165,7 @@ describe('CommentController dismiss-suggestion authz', () => {
       commentRepo,
       pageRepo,
       pageAccessService,
+      spaceAbility,
     };
   }
 
@@ -166,10 +173,12 @@ describe('CommentController dismiss-suggestion authz', () => {
   const workspace: any = { id: 'ws-1' };
   const provenance: any = undefined;
   const dto: any = { commentId: 'c-1' };
+  // Owned by the acting user (u-1) unless a test overrides creatorId.
   const comment = {
     id: 'c-1',
     pageId: 'p-1',
     spaceId: 'sp-1',
+    creatorId: 'u-1',
     suggestedText: 'new text',
     selection: 'old text',
   };
@@ -258,4 +267,58 @@ describe('CommentController dismiss-suggestion authz', () => {
       controller.dismissSuggestion(dto, user, workspace, provenance),
     ).rejects.toBeInstanceOf(BadRequestException);
   });
+
+  // --- #338 owner-or-space-admin gate (mirrors POST /comments/delete) --------
+  // A childless dismiss irreversibly hard-deletes the comment, so canComment is
+  // not enough: only the comment owner or a space admin may dismiss.
+
+  it('owner dismisses their own suggestion → allowed, no admin check needed', async () => {
+    const { controller, commentRepo, pageRepo, commentService, spaceAbility } =
+      makeController(false);
+    // comment.creatorId === user.id (owner).
+    commentRepo.findById.mockResolvedValue(comment);
+    pageRepo.findById.mockResolvedValue(page);
+
+    await controller.dismissSuggestion(dto, user, workspace, provenance);
+
+    // Owner short-circuits the admin lookup.
+    expect(spaceAbility.createForUser).not.toHaveBeenCalled();
+    expect(commentService.dismissSuggestion).toHaveBeenCalledWith(
+      comment,
+      user,
+      provenance,
+    );
+  });
+
+  it('non-owner non-admin → Forbidden AND the service is never called', async () => {
+    const { controller, commentRepo, pageRepo, commentService, spaceAbility } =
+      makeController(false); // NOT a space admin
+    commentRepo.findById.mockResolvedValue({
+      ...comment,
+      creatorId: 'someone-else',
+    });
+    pageRepo.findById.mockResolvedValue(page);
+
+    await expect(
+      controller.dismissSuggestion(dto, user, workspace, provenance),
+    ).rejects.toBeInstanceOf(ForbiddenException);
+
+    expect(spaceAbility.createForUser).toHaveBeenCalledWith(user, comment.spaceId);
+    expect(commentService.dismissSuggestion).not.toHaveBeenCalled();
+  });
+
+  it('non-owner space admin → allowed to dismiss another user’s suggestion', async () => {
+    const { controller, commentRepo, pageRepo, commentService, spaceAbility } =
+      makeController(true); // space admin
+    commentRepo.findById.mockResolvedValue({
+      ...comment,
+      creatorId: 'someone-else',
+    });
+    pageRepo.findById.mockResolvedValue(page);
+
+    await controller.dismissSuggestion(dto, user, workspace, provenance);
+
+    expect(spaceAbility.createForUser).toHaveBeenCalledWith(user, comment.spaceId);
+    expect(commentService.dismissSuggestion).toHaveBeenCalled();
+  });
 });