fix(ws): pin the withheld ACK with a test, and correct the changeset

Two blockers from the pre-submission gate, both reproduced before fixing.

1. The changeset claimed the non-mux POST branch answers OPERATION_FAILED. The
   code says the opposite in as many words ("NOT an error response,
   deliberately"), the commit message says response codes are unchanged, and the
   test asserts the 200. It was a leftover sentence from an earlier iteration that
   would have shipped into the CHANGELOG announcing an API contract change that
   does not exist — and errorCode values are SemVer-relevant per
   docs/versioning-policy.md.

2. The WebSocket half of the fix had no test protection: reverting ws-routes.ts to
   master left all 9 tests green, while the commit message sells "plus the whole
   WebSocket path" as part of the fix. Three tests added against the real WS
   route — ACK on delivery, ACK withheld and seq re-opened when the write did not
   land, and a deduplicated frame still ACKed so the client can drop it. Verified
   the other way round: with ws-routes.ts reverted, the middle one fails.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.changeset/input-delivery-retryable.md

@@ -14,8 +14,11 @@ been delivered.
 
 The bookkeeping is now rolled back on failure and the WebSocket ACK withheld, so
 the client redelivers. `Session.write()` reports whether it reached a PTY at all
-instead of silently swallowing the data, and the non-mux POST branch — whose
-response has not gone out yet — answers `OPERATION_FAILED` rather than a cheerful 200.
+instead of silently swallowing the data.
+
+Response codes are unchanged: a session can legitimately have no PTY yet (created
+but not started), so turning that into a failure status would be a contract change
+of its own.
 
 Note this does not remove the root cause: the POST still answers 200 before the
 mux write is attempted, so a client that treats any 2xx as final still cannot