Disabling per-path keep-alive on the server-side only causes backup paths to be abandoned on the client-side

[HeikoBornholdt]

Describe the bug
In an attempt to unburden our servers, we disabled per-path keep-alive on the server-side only (default_path_keep_alive_interval unset there), so keep-alive is performed by clients only and the server just has to answer.

With this particular setting, we observed a behavior we did not expect: every path opened as PathStatus::Backup gets abandoned as PathAbandonReason::TimedOut once per idle-timeout span, even though its per-path keep-alive pings leave every 5 s, arrive, and are acknowledged, well inside the 15 s per-path idle timeout configured on it.

I'm not that deep inside QUIC multipath internals (for now), so I don't know how ACKs should be processed. But what I observed along the way: while the keep-alives leave on the backup path, the acks arrive on path 0 and appear to be counted as a sign of life of path 0.

This may share a root cause with #799 (all PATH_ACKs ride the path that transmits first). The symptom here is a different one, and as far as I can tell from #801 the proposed fix would not reach a backup path, since a backup path may not send data frames while an available path exists.

Relevant Logs
(I will try to provide a MRE soon)

Expected behavior
Clients perform the keep-alive while the server may stay passive, and healthy paths are not falsely abandoned.

noq

Version: 1.3.0 @ c1f411562

noq v1.3.0 (/Users/bornholdt/Developer/noq/noq)
├── noq feature "bloom"
│   └── noq feature "default" (command-line)
├── noq feature "default" (command-line)
├── noq feature "platform-verifier"
│   └── noq feature "default" (command-line)
├── noq feature "ring"
│   └── noq feature "default" (command-line)
├── noq feature "runtime-tokio"
│   └── noq feature "default" (command-line)
├── noq feature "rustls"
│   └── noq feature "default" (command-line)
└── noq feature "tracing-log"
    └── noq feature "default" (command-line)

Platform(s)
Desktop:

• OS: macOS
• Version 26.6.2

lolwut

• This issue was created by a human that thought critically about
  the issue being reported and wrote this as concisely and clearly
  as they could. Taking full responsibility for the issue being
  accurate.
• This issue isn't slop, and is backed up by evidence.

[HeikoBornholdt]

Here is a minimal reproducible example:
c1f4115...HeikoBornholdt:noq:backup-path-idle-repro

# Terminal 1
$ cargo run --example server -- ./ --listen 127.0.0.1:4433

# Terminal 2
$ RUST_LOG=info cargo run --example client -- --host localhost https://127.0.0.1:4433/Cargo.toml

The backup path is abandoned 20 s after it was established, and the stats of the discarded path show it was alive: 7 pings sent on it, nothing lost, and not a single PATH_ACK received on it.

...
staying connected for 40s
2026-09-11T11:11:43.957306Z  INFO client: path event event=Abandoned { id: PathId(1), reason: TimedOut }
2026-09-11T11:11:44.052107Z  INFO client: path event event=Discarded { id: PathId(1), path_stats: PathStats { rtt: 1.353916ms, udp_tx: UdpStats { datagrams: 9, bytes: 8077, ios: 0 }, udp_rx: UdpStats { datagrams: 5, bytes: 6787, ios: 0 }, frame_tx: FrameStats { ACK: 0, ACK_FREQUENCY: 0, CONNECTION_CLOSE: 0, CRYPTO: 0, DATA_BLOCKED: 0, DATAGRAM: 0, HANDSHAKE_DONE: 0, IMMEDIATE_ACK: 5, MAX_DATA: 0, MAX_PATH_ID: 0, MAX_STREAM_DATA: 0, MAX_STREAMS_BIDI: 0, MAX_STREAMS_UNI: 0, NEW_CONNECTION_ID: 0, NEW_TOKEN: 0, PATHS_BLOCKED: 0, PATH_ABANDON: 0, PATH_ACK: 0, PATH_STATUS_AVAILABLE: 0, PATH_STATUS_BACKUP: 0, PATH_CHALLENGE: 1, PATH_CIDS_BLOCKED: 0, PATH_NEW_CONNECTION_ID: 0, PATH_RESPONSE: 1, PATH_RETIRE_CONNECTION_ID: 0, PING: 7, RESET_STREAM: 0, RETIRE_CONNECTION_ID: 0, STREAM_DATA_BLOCKED: 0, STREAMS_BLOCKED_BIDI: 0, STREAMS_BLOCKED_UNI: 0, STOP_SENDING: 0, STREAM: 0, OBSERVED_ADDRESS: 2, ADD_ADDRESS: 0, REACH_OUT: 0, REMOVE_ADDRESS: 0 }, frame_rx: FrameStats { ACK: 0, ACK_FREQUENCY: 0, CONNECTION_CLOSE: 0, CRYPTO: 0, DATA_BLOCKED: 0, DATAGRAM: 0, HANDSHAKE_DONE: 0, IMMEDIATE_ACK: 5, MAX_DATA: 0, MAX_PATH_ID: 0, MAX_STREAM_DATA: 0, MAX_STREAMS_BIDI: 0, MAX_STREAMS_UNI: 0, NEW_CONNECTION_ID: 0, NEW_TOKEN: 0, PATHS_BLOCKED: 0, PATH_ABANDON: 0, PATH_ACK: 0, PATH_STATUS_AVAILABLE: 0, PATH_STATUS_BACKUP: 0, PATH_CHALLENGE: 1, PATH_CIDS_BLOCKED: 0, PATH_NEW_CONNECTION_ID: 0, PATH_RESPONSE: 1, PATH_RETIRE_CONNECTION_ID: 0, PING: 4, RESET_STREAM: 0, RETIRE_CONNECTION_ID: 0, STREAM_DATA_BLOCKED: 0, STREAMS_BLOCKED_BIDI: 0, STREAMS_BLOCKED_UNI: 0, STOP_SENDING: 0, STREAM: 0, OBSERVED_ADDRESS: 1, ADD_ADDRESS: 0, REACH_OUT: 0, REMOVE_ADDRESS: 0 }, cwnd: 12000, congestion_events: 0, spurious_congestion_events: 0, lost_packets: 0, lost_bytes: 0, sent_plpmtud_probes: 4, lost_plpmtud_probes: 0, black_holes_detected: 0, current_mtu: 1452 } }