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>

internal/daemon/controller/handler.go

@@ -11,10 +11,10 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"github.com/hashicorp/boundary/api"
 	"io"
 	"net/http"
 	"net/textproto"
-	"os"
 	"path"
 	"strings"
 	"time"
@@ -476,7 +476,7 @@ func wrapHandlerWithCommonFuncs(h http.Handler, c *Controller, props HandlerProp
 	}
 
 	disableAuthzFailures := c.conf.DisableAuthorizationFailures ||
-		(c.conf.RawConfig.DevController && os.Getenv("BOUNDARY_DEV_SKIP_AUTHZ") != "")
+		(c.conf.RawConfig.DevController && api.Getenv("BOUNDARY_DEV_SKIP_AUTHZ") != "")
 	if disableAuthzFailures {
 		event.WriteSysEvent(context.TODO(), op, "AUTHORIZATION CHECKING DISABLED")
 	}
@@ -629,7 +629,7 @@ type cmdAttrs struct {
 }
 
 func wrapHandlerWithCallbackInterceptor(h http.Handler, c *Controller) http.Handler {
-	logCallbackErrors := os.Getenv("BOUNDARY_LOG_CALLBACK_ERRORS") != ""
+	logCallbackErrors := api.Getenv("BOUNDARY_LOG_CALLBACK_ERRORS") != ""
 
 	return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
 		const op = "controller.wrapHandlerWithCallbackInterceptor"