fix(eio,eio-client): do not close the connection while packets are still being received

[GiHoon1123]

Summary

Fixes #5450.

The connection could be closed on a ping timeout even while other packets kept arriving, because only the pong (server side) or a fresh ping (client side) reset the timeout - a high-throughput connection could be dropped despite clear evidence it was still alive.

A previous fix for this (be7b4e7) was reverted (5359bae) for two reasons: it allocated a new timer on every incoming packet, and it delayed the next scheduled ping as a side effect (since only the timeout timer was touched, not the interval timer).

This takes a different approach:

• A plain timestamp of the last received packet is tracked (updated on every packet, no timer touched on this path).
• Only when the pre-existing ping-timeout timer actually fires does a check run: if a packet arrived recently enough, the timer is extended by the remaining time instead of closing; otherwise the connection is closed as before.
• The ping/pong schedule itself (pingIntervalTimer on the server, the client's own ping-driven timer) is never touched, so there's no change to when pings are sent.

Both engine.io and engine.io-client needed the change. Delaying the server's wait for a pong also delays the server's next ping; without a matching client-side check, the client's own independent ping-timeout watchdog would still close the connection on its own once that delay exceeded pingTimeout. This was confirmed by testing the server-only fix in isolation - the client still disconnected on its own.

_hasPingExpired(), the synchronous check the Socket.IO client layer uses to detect a throttled/delayed timeout timer, is also updated to apply the same recent-packet check before deciding the connection is stale, since it's a separate path that can bypass the timer-based check entirely.

Test plan

• New test: server does not close the connection when the client's pongs are delayed but other packets keep arriving (client's own watchdog neutralized to isolate server-side behavior)
• New test: client does not close the connection when the server's pings are delayed but other packets keep arriving (server's own ping suppressed to isolate client-side behavior)
• New test: _hasPingExpired() returns false when a packet was recently received, even if the throttled-timer deadline has passed
• Existing engine.io suite passes for both protocol v4 and legacy v3 (EIO_CLIENT=3)
• Existing engine.io-client suite passes
• Verified regression coverage by reverting each fix individually (git stash on just that file) and confirming the corresponding new test fails without it, then restoring and reconfirming it passes

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