fix: increase SSH connection timeout from 5s to 15s for child nodes

[jakub961241]

Summary

Fixes #11621 - Child node SSH connection fails with "ssh: handshake failed: i/o timeout" when network latency is high (e.g., domestic China to overseas servers).

Root Cause

Default DialTimeOut was hardcoded to 5 seconds at core/utils/ssh/ssh.go:51 and agent/utils/ssh/ssh.go:43. For cross-border connections with several seconds of latency, this wasn't enough for both TCP dial + SSH handshake.

Additionally, DialWithTimeout set the handshake deadline equal to the dial timeout (conn.SetDeadline(time.Now().Add(config.Timeout))), which meant the SSH handshake had to complete in the same window as the dial.

Fix

• Increase default DialTimeOut from 5s to 15s (both core and agent)
• Set handshake deadline to 2x the timeout (config.Timeout * 2) to give more time for the SSH key exchange after TCP connection

Changed files

• core/utils/ssh/ssh.go - Default timeout 5s → 15s, handshake deadline × 2
• agent/utils/ssh/ssh.go - Default timeout 5s → 15s

fix: Increase SSH connection timeout from 5s to 15s for better child node connectivity (#11621)

[f2c-ci-robot]

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