Alerting: Surface remote AM silence creation errors properly

When creating silences in remote Alertmanager instances, all 4xx errors were treated as 500s.

This change ensures that 4xx errors are properly surfaced as bad payload errors, allowing callers to handle them appropriately.

pkg/services/ngalert/remote/alertmanager.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/base64"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"hash/fnv"
 	"maps"
@@ -13,6 +14,7 @@ import (
 	"strings"
 	"time"
 
+	openapiRuntime "github.com/go-openapi/runtime"
 	"github.com/go-openapi/strfmt"
 	"github.com/google/go-cmp/cmp"
 	"github.com/google/go-cmp/cmp/cmpopts"
@@ -465,6 +467,18 @@ func (am *Alertmanager) CreateSilence(ctx context.Context, silence *apimodels.Po
 	params := amsilence.NewPostSilencesParamsWithContext(ctx).WithSilence(silence)
 	res, err := am.amClient.Silence.PostSilences(params)
 	if err != nil {
+		// Translate downstream 4xx errors into a well-known bad payload error so callers can surface HTTP 400.
+		// The swagger client returns typed errors and/or an *openapiRuntime.APIError with a Code field.
+		var badReq *amsilence.PostSilencesBadRequest
+		if errors.As(err, &badReq) {
+			return "", fmt.Errorf("%w: %v", alertingNotify.ErrCreateSilenceBadPayload, err)
+		}
+		var apiErr *openapiRuntime.APIError
+		if errors.As(err, &apiErr) {
+			if apiErr.Code >= http.StatusBadRequest && apiErr.Code < http.StatusInternalServerError {
+				return "", fmt.Errorf("%w: %v", alertingNotify.ErrCreateSilenceBadPayload, err)
+			}
+		}
 		return "", err
 	}