Remove unused experimental aitools helpers (#4733)

## Summary
- remove the unused `experimental/aitools` detector, pathutil, and
prompts packages
- inline the auth error formatter in middleware and add focused coverage
for the rendered message
- trim the session package down to the context and key-value helpers
still used by live aitools commands

## Test plan
- [x] `go test ./experimental/aitools/...`

Author: arsenyinfo

experimental/aitools/lib/detector/bundle_detector.go

@@ -3,8 +3,9 @@ package middlewares
 import (
 	"context"
 	"errors"
+	"fmt"
+	"strings"
 
-	"github.com/databricks/cli/experimental/aitools/lib/prompts"
 	"github.com/databricks/cli/experimental/aitools/lib/session"
 	"github.com/databricks/cli/libs/databrickscfg/profile"
 	"github.com/databricks/databricks-sdk-go"
@@ -37,9 +38,20 @@ func GetDatabricksClient(ctx context.Context) (*databricks.WorkspaceClient, erro
 }
 
 func newAuthError(ctx context.Context) error {
-	// Prepare template data
-	data := map[string]any{
-		"Profiles": GetAvailableProfiles(ctx),
+	return errors.New(formatAuthError(GetAvailableProfiles(ctx)))
+}
+
+func formatAuthError(profiles profile.Profiles) string {
+	var b strings.Builder
+	b.WriteString("Not authenticated to Databricks\n\n")
+	b.WriteString("I need to know either the Databricks workspace URL or the Databricks profile name.\n\n")
+	b.WriteString("The available profiles are:\n\n")
+	for _, p := range profiles {
+		fmt.Fprintf(&b, "- %s (%s)\n", p.Name, p.Host)
 	}
-	return errors.New(prompts.MustExecuteTemplate("auth_error.tmpl", data))
+	b.WriteString("\n")
+	b.WriteString("IMPORTANT: YOU MUST ASK the user which of the configured profiles or databricks workspace URL they want to use.\n")
+	b.WriteString("Then set the DATABRICKS_HOST and DATABRICKS_TOKEN environment variables, or use a named profile via DATABRICKS_CONFIG_PROFILE.\n\n")
+	b.WriteString("Do not run anything else before authenticating successfully.\n")
+	return b.String()
 }