fix: hand back the subscription when the sub reply is abandoned or expires

[diegolmello]

Proposed changes

sendSubscription recorded the subscription on the AbandonedRequest / ExpiredWait branch but dropped rememberSubscription's return value, so the caller got undefined for a stream the registry genuinely held. With no handle there was no unsub to send, and the orphaned entry was re-established on every reconnect for the rest of the session.

The branch now returns the recorded subscription, establishing the invariant that a handle exists exactly when a registry entry exists. The two adjacent paths already satisfy it: a reply that carries no subs, and a subscribe with no live connection, both write no entry and return nothing.

Closes #386

Steps to reproduce

• Subscribe to a stream.
• Have the sub reply be abandoned, or its wait expire, while the connection is still installed.
• Observe the returned handle is undefined while the subscription registry holds the entry.
• Attempt teardown: with no handle there is no unsub to send.
• Reconnect: the orphaned entry is resubscribed.

Tests

• tears the abandoned entry down through the handle alone
• is the entry itself when the deadline expires
• is withheld only where no entry was written
• updated the pinned assertions for the reopen-abandoned, socket-closed, and deadline-expired paths, which previously pinned the missing handle