fix(gateway): honour on_disconnect on /wait endpoints (#3267)

* fix(gateway): honour on_disconnect on /wait endpoints (#3265)

The non-streaming /threads/{tid}/runs/wait and /runs/wait handlers used
to await record.task directly with no disconnect handling and silently
swallow CancelledError. When a long tool call (e.g. pip install inside
a custom skill) kept the connection idle long enough for an
intermediate HTTP layer to time out, the handler would still read the
in-progress checkpoint and return it as if the run had completed
normally -- masking a half-finished run as a successful response.

Add wait_for_run_completion in app.gateway.services that mirrors
sse_consumer's bridge-consumption pattern: subscribe to the stream
bridge until END_SENTINEL, poll request.is_disconnected on every
wake-up, and on real client disconnect cancel the background run when
record.on_disconnect is "cancel". Wire it into both wait endpoints.

The streaming path was unaffected because sse_consumer already has
this loop; this just brings /wait to parity.

* fix(gateway): skip checkpoint serialization on /wait disconnect

Copilot review on #3267 caught a follow-on of the same #3265 bug: when
the client disconnects, wait_for_run_completion breaks out of the bridge
loop and cancels the run, but the /wait endpoint then continues to read
the checkpointer and serializes whatever partial checkpoint exists as a
normal 200 response.

Have the helper return a bool — True only when END_SENTINEL was observed
— and skip the checkpoint serialization path on False. Also reorder the
inner check so END_SENTINEL is honoured even when is_disconnected() flips
true in the same iteration; the run truly finished so the real final
checkpoint is still valid.