Add THRESHOLD_* env var support with BOUNDARY_* fallback

Introduce api.Getenv/api.LookupEnv which prefer THRESHOLD_X and fall back to legacy BOUNDARY_X; use them for user-facing env var reads.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

api/output_string.go

@@ -5,7 +5,6 @@ package api
 
 import (
 	"fmt"
-	"os"
 	"strings"
 
 	retryablehttp "github.com/hashicorp/go-retryablehttp"
@@ -60,8 +59,8 @@ func (d *OutputStringError) parseRequest() {
 	for k, v := range d.Request.Header {
 		for _, h := range v {
 			if strings.ToLower(k) == "authorization" {
-				tokenName := os.Getenv("BOUNDARY_TOKEN_NAME")
-				keyringType := os.Getenv("BOUNDARY_KEYRING_TYPE")
+				tokenName := Getenv("BOUNDARY_TOKEN_NAME")
+				keyringType := Getenv("BOUNDARY_KEYRING_TYPE")
 				switch {
 				case tokenName == "none" || keyringType == "none":
 					h = `Bearer <token>`