Preserve pending CP HiCache backups through final insert

Deferred non-chunked inserts can happen while a CP HiCache per-layer backup is still in flight. Rolling back the unattached prepared backup at unfinished-request time drops the only reservation that final insertion can attach, so cache_finished_req has to start a post-forward backup and catch up all layers synchronously.\n\nKeep the prepared backup for non-chunked deferred inserts and continue rolling it back for chunked middle inserts, where the suffix may be extended by later chunks and the old backup would cover the wrong range. Document the failure mode so future changes do not rediscover the same fallback path.\n\nConstraint: CP HiCache radix split cannot mutate in-flight backup nodes.\nConstraint: Chunked prefill middle inserts still need rollback because their backup range is not final.\nRejected: Always rollback unattached backups | causes post-forward catch_up_all_layers for non-chunked deferred inserts.\nRejected: Always preserve unattached backups | can attach stale backup ranges for chunked middle inserts.\nConfidence: high\nScope-risk: narrow\nDirective: Do not clear req.cp_hicache_prepared_backup on non-chunked deferred insert without proving final insert no longer needs it.\nTested: remote cjy-glm5-new py_compile radix_cache.py\nTested: remote cjy-glm5-new pytest test_cp_hicache_metadata.py::{nonchunked preserve,chunked rollback} => 2 passed\nNot-tested: live ETE replay after restarting prefill with this exact commit

python/sglang/srt/mem_cache/radix_cache.py

@@ -725,7 +725,15 @@ class RadixCache(BasePrefixCache):
                 prepared_cp_backup is not None
                 and not getattr(prepared_cp_backup, "attached", False)
                 and hasattr(self, "_rollback_prepared_cp_backup")
+                and chunked
             ):
+                # A chunked middle insert can be followed by another forward for
+                # the same request, so an unattached prepared reservation would
+                # cover the wrong suffix and block the next chunk from preparing
+                # its own backup.  Non-chunked disagg prefill still has a later
+                # cache_finished_req() pass after KV transfer; keep the prepared
+                # backup alive so that final insert can attach it instead of
+                # falling back to post-forward all-layer catch-up backup.
                 self._rollback_prepared_cp_backup(
                     prepared_cp_backup, "unfinished_pending_backup_split_deferred"
                 )