Give prefill inflight transfers a liveness bound

E2e caught one request wedged FOREVER in disagg_prefill_inflight_queue
(probes 21s apart with zero traffic both showed #inflight-req: 1, no
reap/timeout warnings ever logged).  Mechanism, established by reading
the full state machine: prefill Success is set locally by the transfer
worker on the LAST chunk; if the decode peer is torn down between the
handshake and the prefill's final send(), add_transfer_request silently
drops the chunk (no transfer destinations) — Success becomes
unreachable.  The only external rescue, the decode ABORT notification,
is best-effort (silently swallowed on send error, no-op if it races the
room registration), there is no prefill-side heartbeat of decode
sessions, and the sender's only timeout covers Bootstrapping — the
inflight queue itself has no liveness bound.  The orphan pins the
request's KV pages and rides every poll collective.

Two fixes, both reaped through the existing Failed branch via the
CP/TP MIN-reduce poll consensus (Failed=0 wins, so one rank concluding
flips every rank together — rank-uniform by construction):

- add_transfer_request: a room with no transfer destinations that is
  NOT already Success (the dummy-rank handshake marking) now concludes
  Failed loudly instead of dropping the chunk silently.
- Inflight residency timeout: entries stuck in a non-terminal poll
  state past SGLANG_DISAGGREGATION_INFLIGHT_TIMEOUT (default 300s,
  matching the sibling BOOTSTRAP/WAITING timeouts) get sender.abort()
  and reap on the next poll.  Covers what the hardening cannot: lost
  ABORT datagrams, decode crashes.

Known sibling gaps left for follow-up: the decode transfer queue has
no Transferring liveness bound, and an abort that matches no queue is
still a silent no-op (much narrower race than first thought — work
requests are ordered before control requests within a tick).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

python/sglang/srt/environ.py

@@ -296,6 +296,10 @@ class Envs:
     SGLANG_DISAGGREGATION_HEARTBEAT_INTERVAL = EnvFloat(5.0)
     SGLANG_DISAGGREGATION_HEARTBEAT_MAX_FAILURE = EnvInt(2)
     SGLANG_DISAGGREGATION_WAITING_TIMEOUT = EnvInt(300)
+    # Liveness bound for entries in the prefill inflight queue (transfer
+    # submitted, waiting to conclude).  300 matches the bootstrap/waiting
+    # timeouts; transfers legitimately take seconds, so do not go below ~60.
+    SGLANG_DISAGGREGATION_INFLIGHT_TIMEOUT = EnvInt(300)
     SGLANG_DISAGGREGATION_NIXL_BACKEND = EnvStr("UCX")
     SGLANG_DISAGGREGATION_ALL_CP_RANKS_TRANSFER = EnvBool(False)
     SGLANG_DISAGGREGATION_TRANSFER_STATS = EnvBool(False)