fix(realtime): restart timeout after resending push

[hsusul]

Summary

• clear a completed push timeout task before dispatching timeout callbacks
• verify a resent push gets a fresh timeout window

Problem

After a push times out, AsyncPush.resend() sends it again but does not schedule another timeout because the completed task is still stored in timeout_task. For channel join retries, a second unacknowledged join can therefore leave the channel waiting indefinitely.

Solution

Release the timeout task reference when the timer fires so resend() can arm a new timer. This keeps cancellation behavior and the public API unchanged.

Tests

• uv run --package realtime pytest src/realtime/tests/test_push.py src/realtime/tests/test_timer.py -q — 7 passed
• make realtime.tests — mypy passed; 32 tests passed against the local Supabase stack
• uv run ruff check src/realtime — passed
• uv run ruff format --check src/realtime — passed
• uv build --package realtime — passed
• git diff --check — passed

Closes #1653

@olirice @silentworks