Replace string matching with syscall error checking for connection resets

[Copilot]

The connection reset detection relied on string matching against platform-specific error messages ("connection was forcibly closed by the remote host" on Windows, "connection reset by peer" on Unix). This is fragile and breaks across Go versions or platforms.

Changes:

• Check underlying syscall error type using errors.Is(opErr.Err, syscall.ECONNRESET) instead of string matching
• Remove strings import, add syscall import
• Platform-agnostic: handles ECONNRESET (Unix) and WSAECONNRESET (Windows) automatically

Before:

if strings.Contains(opErr.Error(), "connection was forcibly closed by the remote host") || 
   strings.Contains(opErr.Error(), "connection reset by peer") {
    logDebugf("Ignoring forceful connection closure by server")
    break
}

After:

if errors.Is(opErr.Err, syscall.ECONNRESET) {
    logDebugf("Ignoring forceful connection closure by server")
    break
}

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[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• hq.documatrix.com
  • Triggering command: /tmp/go-build4141503950/b001/go-lprlib.test /tmp/go-build4141503950/b001/go-lprlib.test -test.testlogfile=/tmp/go-build4141503950/b001/testlog.txt -test.paniconexit0 -test.timeout=10m0s -test.v=true (dns block)
  • Triggering command: /tmp/go-build2398867556/b001/go-lprlib.test /tmp/go-build2398867556/b001/go-lprlib.test -test.testlogfile=/tmp/go-build2398867556/b001/testlog.txt -test.paniconexit0 -test.timeout=10m0s -test.v=true -test.run=TestDNS (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)