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>

src/web/routes/case-routes.ts

@@ -1717,7 +1717,8 @@ export function registerCaseRoutes(
    * The tab group new sessions of this case land in when the create request names
    * no group and no parent places them. Scoped to the caller's own tab layout: the
    * options are their groups, and a stored id that is no longer one of them reads
-   * as Ungrouped (null) and is forgotten. Not admin-gated in multi-user mode: the
+   * as Ungrouped (null); the read writes nothing (the next creation for the case
+   * forgets it). Not admin-gated in multi-user mode: the
    * value is per owner (like the layout it points into), never server-wide.
    */
   app.get('/api/cases/:name/default-group', async (req, reply) => {