[Backport staging] Send Content-Type: application/json header for integration manager /register API (#9491)

Co-authored-by: Michael Telatynski <7t3chguy@gmail.com>

src/ScalarAuthClient.ts

@@ -190,6 +190,9 @@ export default class ScalarAuthClient {
         const res = await fetch(scalarRestUrl, {
             method: "POST",
             body: JSON.stringify(openidTokenObject),
+            headers: {
+                "Content-Type": "application/json",
+            },
         });
 
         if (!res.ok) {