feat: Add a command to stop the local stack

[nfebe]

Add a stop command for the local stack. Requires an agent release with shutdown support.

[sourceant-local]

Code Review Summary

This revision implements the requested sourceant stop support. The agent client now sends Accept: application/json, handles the stop response with a switch, and exposes IsConnectionRefused. The command uses that helper, and tests cover success, unsupported status codes, and timeout behavior.

🚀 Key Improvements

• Refactored internal/agent/client.go to use a switch for status-code handling, as previously suggested.
• Added Accept: application/json to the stop request for consistency with other client methods.
• Added agent.IsConnectionRefused so the command package no longer imports syscall directly.
• Strengthened internal/command/stop_test.go to assert the unsupported-agent error message and added a timeout-vs-refused test.

💡 Minor Suggestions

• Clarify the comment in internal/agent/client.go so it covers 200, 404, and 405.
• Add a doc comment for the exported IsConnectionRefused helper.
• Tighten the timeout test assertion to check for the specific error substring.

[sourceant-local]

Review complete. See the overview comment for a summary.

[sourceant-local]

Review complete. See the overview comment for a summary.

[sourceant-local]

The comment only explains the 200 case, but this branch also groups 404 and 405. Update it to describe all three unsupported statuses so future readers don't wonder why NotFound/MethodNotAllowed are handled here.

Suggested change

	case http.StatusOK, http.StatusNotFound, http.StatusMethodNotAllowed:
	case http.StatusOK, http.StatusNotFound, http.StatusMethodNotAllowed:
	// Only 204 acknowledges shutdown; 200/404/405 mean this agent does not support stop.

[sourceant-local]

Exported functions should have a doc comment explaining their contract, especially since this is a new public helper used by the command package.

Suggested change

	func IsConnectionRefused(err error) bool {
	// IsConnectionRefused reports whether err represents an agent that is not
	// listening because the connection was refused.
	func IsConnectionRefused(err error) bool {
	var unreachable *Unreachable
	return errors.As(err, &unreachable) && errors.Is(unreachable.Cause, syscall.ECONNREFUSED)
	}

[sourceant-local]

Checking that stderr is non-empty is weak. Assert the actual error message so the test fails with a clear diagnostic if the wrong error path is taken (for example, if a future change prints "already stopped" to stderr instead of stdout).

Suggested change

	if strings.Contains(out.String(), "already stopped") || stderr.Len() == 0 {
	if strings.Contains(out.String(), "already stopped") || !strings.Contains(stderr.String(), "could not stop SourceAnt") {
	t.Fatalf("stdout=%s stderr=%s", out.String(), stderr.String())
	}