fix disconnect event in tcp connector

[ltitanb]

Summary

Fix TCP disconnect reporting for connections that are dropped outside read-side event handling.

Issue

TcpConnector only emitted PollEvent::Disconnect when a connection was found dead while processing a mio event. Connections can also be dropped during send-side work:

• broadcast writes that fail immediately
• single-target writes that fail immediately
• send backlog flushes that fail
• backlog limit enforcement

Those paths removed the connection from ConnectionManager internally, but the caller never received a disconnect event. Callers that maintain their own token-to-peer state could then keep stale connection state forever.

Root Cause

disconnect_at_index was called directly from write/backlog paths without notifying the application. Only read-side polling called the user's handler with PollEvent::Disconnect.

Accepted inbound sockets also did not get TCP_USER_TIMEOUT, while outbound sockets did. That made unclean inbound connection loss slower to detect under network failure.

Fix

• Add a pending-disconnect queue to ConnectionManager.
• Route send-side and backlog-side connection drops through a helper that records the token before removing the connection.
• Drain pending disconnects through poll_with and poll_with_produce, so callers receive the same PollEvent::Disconnect regardless of where the dead socket was detected.
• Apply TCP_USER_TIMEOUT to accepted inbound sockets as well as outbound sockets.

This preserves the existing public API while fixing the event stream seen by callers.

Validation

• Added backlog_disconnect_is_reported, which forces a backlog-triggered disconnect and asserts that PollEvent::Disconnect is emitted for the accepted stream token.
• Ran cargo test -p flux-network.
• Ran cargo clippy -p flux-network --all-targets -- -D warnings.
• Ran git diff --check.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 4 additional findings.