fix(tabs): keep the case-default GET read-only, forget a stale id outside the rollback

Merge-time fix for #676 (review nits 4 and 5). sessionCreated forgot a stale
case default inside the try whose catch rolls the registration back; it now
does so after the try, once the session is committed, so the cleanup can
never drop a committed session from the live map. caseDefaultGroup (behind
GET /api/cases/:name/default-group) no longer deletes a stale id as a side
effect: it reports Ungrouped and leaves the value for the next creation to
forget. The two tests that expected the read to clear it now expect it to
write nothing; the creation test still pins the cleanup.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test/case-default-group.test.ts

@@ -299,7 +299,7 @@ describe('TabLayoutService case default placement', () => {
 });
 
 describe('TabLayoutService case default reads and writes', () => {
-  it('reports the selection with the owner group options, clearing a stale id', async () => {
+  it('reports the selection with the owner group options, and a stale id as Ungrouped without writing', async () => {
     const h = createHarness({ layouts: { alice: grouped() }, live: facts() });
     expect(await h.service.caseDefaultGroup('alice', 'app')).toEqual({
       groupId: null,
@@ -310,7 +310,9 @@ describe('TabLayoutService case default reads and writes', () => {
     });
     h.defaults.alice = { app: 'gone' };
     expect((await h.service.caseDefaultGroup('alice', 'app')).groupId).toBeNull();
-    expect(h.caseDefaults.getCaseDefaultGroup('alice', 'app')).toBeUndefined();
+    // A read never writes: the next creation for the case is what forgets the stale id.
+    expect(h.caseDefaults.deleteCaseDefaultGroup).not.toHaveBeenCalled();
+    expect(h.caseDefaults.getCaseDefaultGroup('alice', 'app')).toBe('gone');
   });
 
   it('saves a known group or null, and refuses an id that is not in the owner layout', async () => {

test/routes/case-default-group-routes.test.ts

@@ -343,10 +343,11 @@ describe('case default group: GET/PUT /api/cases/:name/default-group', () => {
     expect(data.groups[1]).toEqual({ id: 'g-ops', name: 'Operations' });
   });
 
-  it('a deleted group reads back as Ungrouped and is cleared', async () => {
+  it('a deleted group reads back as Ungrouped, and the read changes nothing', async () => {
     world.defaults[OWNER] = { [CASE]: 'g-gone' };
     expect((await get()).json().data.groupId).toBeNull();
-    expect(CASE in world.defaults[OWNER]).toBe(false);
+    // GET is read-only; the next session created for the case forgets the stale id.
+    expect(world.defaults[OWNER][CASE]).toBe('g-gone');
   });
 
   it.each([