[client] Move the MDM precedence into ResolveUploadURL

Three of the four bundle paths ordered the destinations themselves, each with
its own copy of "MDM first, then what was asked for". Give the resolver the MDM
value as its first argument so the order lives in one place and a call site
cannot express a different one.

Pure refactor: the daemon passes "" for now, matching what it does today, and
the remote job and both mobile SDKs resolve exactly as before.

client/internal/debug/destination_test.go

@@ -14,12 +14,29 @@ func TestResolveUploadURL(t *testing.T) {
 		requestedURL = "https://requested.example.com/upload-url"
 	)
 
+	const mdmURL = "https://mdm.example.com/upload-url"
+
 	tests := []struct {
 		name      string
+		mdm       string
 		requested string
 		published string
 		want      string
 	}{
+		{
+			// Pinning the destination on a managed device is pointless if the
+			// person at the keyboard can name another one.
+			name:      "MDM outranks a URL the caller named",
+			mdm:       mdmURL,
+			requested: requestedURL,
+			published: operatorURL,
+			want:      mdmURL,
+		},
+		{
+			name: "MDM alone wins",
+			mdm:  mdmURL,
+			want: mdmURL,
+		},
 		{
 			name:      "requested wins over published",
 			requested: requestedURL,
@@ -47,7 +64,7 @@ func TestResolveUploadURL(t *testing.T) {
 
 	for _, tc := range tests {
 		t.Run(tc.name, func(t *testing.T) {
-			assert.Equal(t, tc.want, ResolveUploadURL(tc.requested, tc.published))
+			assert.Equal(t, tc.want, ResolveUploadURL(tc.mdm, tc.requested, tc.published))
 		})
 	}
 }