[AGENTCFG-1] The diagnose command should error if an API key is not configured (#46942)

### What does this PR do?
When running the `diagnose` command without an API key configured the command exit successfully saying that no check where run.

We want to fail gracefully with an error message.

### Motivation

### Describe how you validated your changes

Added a unit test, and ran the agent with/without an api key set. The error appears when running the `diagnose` command without an api key.

### Additional Notes


Co-authored-by: rahul.kaukuntla <rahul.kaukuntla@datadoghq.com>

Author: rahulkaukuntla

cmd/agent/subcommands/diagnose/command.go

@@ -9,6 +9,7 @@ package diagnose
 import (
 	"bytes"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"os"
 	"sort"
@@ -417,6 +418,11 @@ func cmdDiagnose(cliParams *cliParams,
 		return nil
 	}
 
+	// API key is required to run diagnose; without it no checks can run and the command would exit successfully with no useful output
+	if !config.IsConfigured("api_key") {
+		return errors.New("no API key configured: diagnose requires an API key to run checks. Set the API key in datadog.yaml or use the DD_API_KEY environment variable")
+	}
+
 	// Run command
 	var err error
 	var result *diagnose.Result

cmd/agent/subcommands/diagnose/command_test.go

@@ -6,8 +6,11 @@
 package diagnose
 
 import (
+	"os"
+	"path/filepath"
 	"testing"
 
+	"github.com/spf13/cobra"
 	"github.com/stretchr/testify/require"
 
 	"github.com/DataDog/datadog-agent/cmd/agent/command"
@@ -23,6 +26,25 @@ func TestDiagnoseCommand(t *testing.T) {
 		func(_ *cliParams, _ core.BundleParams) {})
 }
 
+func TestDiagnoseCommandFailsWithoutAPIKey(t *testing.T) {
+	t.Setenv("DD_API_KEY", "")
+
+	dir := t.TempDir()
+	configPath := filepath.Join(dir, "datadog.yaml")
+	require.NoError(t, os.WriteFile(configPath, []byte("hostname: test\n"), 0o600))
+
+	root := &cobra.Command{Use: "agent"}
+	for _, c := range Commands(&command.GlobalParams{ConfFilePath: dir}) {
+		root.AddCommand(c)
+	}
+	root.SetArgs([]string{"diagnose"})
+
+	err := root.Execute()
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "no API key configured")
+	require.Contains(t, err.Error(), "DD_API_KEY")
+}
+
 func TestShowMetadataV5Command(t *testing.T) {
 	fxutil.TestOneShotSubcommand(t,
 		Commands(&command.GlobalParams{}),

releasenotes/notes/diagnose-command-returns-error-API-key-is-not-set-3fb35261884b9930.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    The diagnose command now returns an error if an API key is not configured.
+