fix: clear pendingHeartbeatRef unconditionally in onConnOpen

[g0rdonL]

Bug

onConnOpen() only clears a stranded pendingHeartbeatRef when autoSendHeartbeat is true (it happens inside the resetHeartbeat() call, which is itself gated on that flag). Clients configured with autoSendHeartbeat: false — notably @supabase/realtime-js's RealtimeClient in worker: true mode, a normal production configuration where heartbeats are driven by a Worker instead of the socket's own timer — never get that clear.

Consequence: a pendingHeartbeatRef armed on one connection and left pending by a drop-with-beat-in-flight (network blip, laptop sleep, server-side close) survives onto the next, healthy connection. That connection's first externally-driven heartbeat then sees the stale non-null ref, takes the timeout branch in heartbeatTimeout(), and force-closes the still-healthy socket with "heartbeat timeout" having sent zero frames.

Filed upstream as supabase/supabase-js#2625 (realtime-js consumes this library and surfaces the bug there — repro included, verified there against the published @supabase/realtime-js/@supabase/phoenix packages before tracing it to this file).

Fix

Hoist the pendingHeartbeatRef/heartbeatSentAt clear out of the autoSendHeartbeat gate in onConnOpen() so it always runs on connection open — matching what resetHeartbeat() already does unconditionally for the autoSendHeartbeat: true path. Only the timer-(re)arming call stays gated, which is unaffected since worker-mode clients drive heartbeats externally.

Verification

• Added a regression test (onConnOpen › clears a stale pendingHeartbeatRef even when autoSendHeartbeat is false) that fails against unfixed main (confirmed) and passes with this change.
• Full suite: npm test → 217/220 passing (3 pre-existing skips), up from 216/219 before the new test.
• npm run lint clean.
• Also re-ran the standalone repro from the linked issue (fake WebSocket + fake Worker driving worker: true mode end-to-end through @supabase/realtime-js) against the published packages to confirm the observed behavior before tracing the root cause here.