fix: potential panics in login and return proper http 405 (#8065)

# Which Problems Are Solved

We identified some parts in the code, which could panic with a nil
pointer when accessed without auth request.
Additionally, if a GRPC method was called with an unmapped HTTP method,
e.g. POST instead of GET a 501 instead of a 405 was returned.

# How the Problems Are Solved

- Additional checks for existing authRequest
- custom http status code mapper for gateway

# Additional Changes

None.

# Additional Context

- noted internally in OPS

internal/api/grpc/server/gateway.go

@@ -10,9 +10,11 @@ import (
 	"github.com/grpc-ecosystem/grpc-gateway/v2/runtime"
 	"github.com/zitadel/logging"
 	"google.golang.org/grpc"
+	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/credentials"
 	"google.golang.org/grpc/credentials/insecure"
 	healthpb "google.golang.org/grpc/health/grpc_health_v1"
+	"google.golang.org/grpc/status"
 	"google.golang.org/protobuf/encoding/protojson"
 	"google.golang.org/protobuf/proto"
 
@@ -36,6 +38,23 @@ var (
 		},
 	}
 
+	httpErrorHandler = runtime.RoutingErrorHandlerFunc(
+		func(ctx context.Context, mux *runtime.ServeMux, marshaler runtime.Marshaler, w http.ResponseWriter, r *http.Request, httpStatus int) {
+			if httpStatus != http.StatusMethodNotAllowed {
+				runtime.DefaultRoutingErrorHandler(ctx, mux, marshaler, w, r, httpStatus)
+				return
+			}
+
+			// Use HTTPStatusError to customize the DefaultHTTPErrorHandler status code
+			err := &runtime.HTTPStatusError{
+				HTTPStatus: httpStatus,
+				Err:        status.Error(codes.Unimplemented, http.StatusText(httpStatus)),
+			}
+
+			runtime.DefaultHTTPErrorHandler(ctx, mux, marshaler, w, r, err)
+		},
+	)
+
 	serveMuxOptions = []runtime.ServeMuxOption{
 		runtime.WithMarshalerOption(jsonMarshaler.ContentType(nil), jsonMarshaler),
 		runtime.WithMarshalerOption(mimeWildcard, jsonMarshaler),
@@ -43,6 +62,7 @@ var (
 		runtime.WithIncomingHeaderMatcher(headerMatcher),
 		runtime.WithOutgoingHeaderMatcher(runtime.DefaultHeaderMatcher),
 		runtime.WithForwardResponseOption(responseForwarder),
+		runtime.WithRoutingErrorHandler(httpErrorHandler),
 	}
 
 	headerMatcher = runtime.HeaderMatcherFunc(