Fix CI failures: golangci-lint config and test port conflicts

[Copilot]

CI was failing due to golangci-lint configuration using deprecated options and flaky tests from port conflicts when packages ran in parallel.

Changes

• golangci-lint config: Replace deprecated skip-dirs with skip-dirs-use-default: true

• Test isolation: Change full_sync tests to port 9080 (avoiding collision with iblt on 8080)

• Connection handling:

  • Add SO_REUSEADDR via net.ListenConfig to eliminate TIME_WAIT port reuse delays
  • Add nil checks in Close() to prevent panics on uninitialized connections

lc := &net.ListenConfig{
    Control: func(network, address string, c syscall.RawConn) error {
        var sockOptErr error
        if err := c.Control(func(fd uintptr) {
            sockOptErr = syscall.SetsockoptInt(int(fd), syscall.SOL_SOCKET, syscall.SO_REUSEADDR, 1)
        }); err != nil {
            return err
        }
        return sockOptErr
    },
}

The SO_REUSEADDR change allows immediate port rebinding, critical for test suite reliability when running packages concurrently.

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