fix(version): only stop port-forwarding when successfully connected

[FalcoSuessgott]

rel. #868 (comment)

> go run cli/cmd/kagent/*.go version
{"backend_version":"unknown","build_date":"unknown","git_commit":"none","kagent_version":"dev"}

kagent versions portforwarding differs now to the other subcommands, perhaps we should just change Stop() to:

func (p *PortForward) Stop() {
  if p == nil {
    return 
  }

	p.cancel()

	if p.cmd.Process != nil {
		p.cmd.Process.Kill() //nolint:errcheck
	}
}

[Copilot]

Pull Request Overview

This PR fixes a bug in the kagent main function where port-forwarding would be stopped even when it failed to initialize properly. The fix ensures port-forwarding cleanup only occurs when the connection is successfully established.

• Only defer pf.Stop() when cli.NewPortForward succeeds
• Prevents potential nil pointer dereference or unnecessary cleanup operations

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