Add bundle debug list-targets command (#5005)

## Why

Provide a lightweight way to retrieve bundle target names along with
their `default`, `mode`, and `workspace.host` fields. This is a
prerequisite for running other bundle CLI commands (`deploy`,
`validate`, `run`, etc.), which all require `--target`.

## Changes

Adds a new hidden `bundle debug list-targets` subcommand that loads only
the local YAML configuration (`bundle.MustLoad` + `phases.Load`) and
returns each target's `name`, `default`, `mode`, and `workspace.host`.
No target selection, no variable resolution, no authentication, no API
calls.

Text output:
  dev (default) development https://dev.example.com
  prod production https://prod.example.com
  staging https://staging.example.com

JSON output:
```json
  {
    "targets": [
      {"name": "dev", "default": true, "mode": "development", "host": "https://dev.example.com"},
      {"name": "prod", "mode": "production", "host": "https://prod.example.com"},
      {"name": "staging", "host": "https://staging.example.com"}
    ]
  }
```

## Test plan

- Unit tests for collectTargets (sorting, all fields, nil workspace,
empty map)
  - Acceptance test covering text and JSON output
  - Command is hidden from bundle debug --help

Author: pavloKozlov

acceptance/bundle/debug/list-targets/databricks.yml

@@ -0,0 +1,27 @@
+bundle:
+  name: test-list-targets
+
+variables:
+  is_default:
+    default: false
+  target_mode:
+    default: production
+  target_host:
+    default: https://var.example.com
+
+targets:
+  dev:
+    default: true
+    mode: development
+    workspace:
+      host: https://dev.example.com
+  staging: {}
+  prod:
+    mode: production
+    workspace:
+      host: https://prod.example.com
+  with_vars:
+    default: ${var.is_default}
+    mode: ${var.target_mode}
+    workspace:
+      host: ${var.target_host}

acceptance/bundle/debug/list-targets/out.test.toml

@@ -0,0 +1,31 @@
+
+>>> [CLI] bundle debug list-targets
+dev (default) development https://dev.example.com
+prod production https://prod.example.com
+staging
+with_vars ${var.target_mode} ${var.target_host}
+
+>>> [CLI] bundle debug list-targets --output json
+{
+  "targets": [
+    {
+      "name": "dev",
+      "default": true,
+      "mode": "development",
+      "host": "https://dev.example.com"
+    },
+    {
+      "name": "prod",
+      "mode": "production",
+      "host": "https://prod.example.com"
+    },
+    {
+      "name": "staging"
+    },
+    {
+      "name": "with_vars",
+      "mode": "${var.target_mode}",
+      "host": "${var.target_host}"
+    }
+  ]
+}

acceptance/bundle/debug/list-targets/output.txt

@@ -0,0 +1,2 @@
+trace $CLI bundle debug list-targets
+trace $CLI bundle debug list-targets --output json

acceptance/bundle/debug/list-targets/script

@@ -17,5 +17,6 @@ func newDebugCommand() *cobra.Command {
 	cmd.AddCommand(debug.NewRefSchemaCommand())
 	cmd.AddCommand(debug.NewStatesCommand())
 	cmd.AddCommand(debug.NewRenderTemplateSchemaCommand())
+	cmd.AddCommand(debug.NewListTargetsCommand())
 	return cmd
 }

cmd/bundle/debug.go

@@ -0,0 +1,107 @@
+package debug
+
+import (
+	"encoding/json"
+	"fmt"
+	"maps"
+	"slices"
+	"strings"
+
+	"github.com/databricks/cli/bundle"
+	"github.com/databricks/cli/bundle/config"
+	"github.com/databricks/cli/bundle/phases"
+	"github.com/databricks/cli/cmd/root"
+	"github.com/databricks/cli/libs/cmdio"
+	"github.com/databricks/cli/libs/diag"
+	"github.com/databricks/cli/libs/flags"
+	"github.com/databricks/cli/libs/logdiag"
+	"github.com/spf13/cobra"
+)
+
+type targetInfo struct {
+	Name    string      `json:"name"`
+	Default bool        `json:"default,omitempty"`
+	Mode    config.Mode `json:"mode,omitempty"`
+	Host    string      `json:"host,omitempty"`
+}
+
+type listTargetsOutput struct {
+	Targets []targetInfo `json:"targets"`
+}
+
+func collectTargets(targets map[string]*config.Target) []targetInfo {
+	names := slices.Sorted(maps.Keys(targets))
+
+	result := make([]targetInfo, 0, len(names))
+	for _, name := range names {
+		t := targets[name]
+		info := targetInfo{
+			Name:    name,
+			Default: t.Default,
+			Mode:    t.Mode,
+		}
+		if t.Workspace != nil {
+			info.Host = t.Workspace.Host
+		}
+		result = append(result, info)
+	}
+	return result
+}
+
+// NewListTargetsCommand returns a command that lists all bundle targets
+// with their name, default, mode, and workspace host fields.
+func NewListTargetsCommand() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:    "list-targets",
+		Short:  "List all available bundle targets",
+		Args:   root.NoArgs,
+		Hidden: true,
+	}
+
+	cmd.RunE = func(cmd *cobra.Command, args []string) error {
+		ctx := logdiag.InitContext(cmd.Context())
+		cmd.SetContext(ctx)
+		logdiag.SetSeverity(ctx, diag.Warning)
+
+		b := bundle.MustLoad(ctx)
+		if b == nil || logdiag.HasError(ctx) {
+			return root.ErrAlreadyPrinted
+		}
+
+		phases.Load(ctx, b)
+		if logdiag.HasError(ctx) {
+			return root.ErrAlreadyPrinted
+		}
+
+		targets := collectTargets(b.Config.Targets)
+
+		switch root.OutputType(cmd) {
+		case flags.OutputText:
+			for _, t := range targets {
+				parts := []string{t.Name}
+				if t.Default {
+					parts = append(parts, "(default)")
+				}
+				if t.Mode != "" {
+					parts = append(parts, string(t.Mode))
+				}
+				if t.Host != "" {
+					parts = append(parts, t.Host)
+				}
+				cmdio.LogString(ctx, strings.Join(parts, " "))
+			}
+		case flags.OutputJSON:
+			buf, err := json.MarshalIndent(listTargetsOutput{Targets: targets}, "", "  ")
+			if err != nil {
+				return err
+			}
+			_, _ = cmd.OutOrStdout().Write(buf)
+		default:
+			return fmt.Errorf("unknown output type %s", root.OutputType(cmd))
+		}
+
+		return nil
+	}
+
+	return cmd
+}