fix(bigtable): check sessionTable.closed in dispatch after lazyPool is warmed - #20913

Summary

• Check t.closed.Load() at entry to sessionTable's dispatch before calling spec.pool.get().
• Previously, guardOpen only checked t.closed during the one-shot lazyPool initialization, so ReadRow / MutateRow calls after sessionTable.Close() on an already-warmed sessionTable bypassed ErrClientClosed and dispatched into the closed invoker.
• Add TestSessionTable_PostCloseAfterLazyOpenReturnsErrClientClosed unit test.