docs: align Bruno tests with Go integration test credentials

- Update Login User.bru to use testuser@example.com
- Update Register User.bru to use testuser@example.com
- Update Register Admin User.bru to use maxdevices@example.com
- Standardize password to Test@Pass123! across all tests
- Add TEST_DATA.md documenting shared test credentials
- Update bruno.json with documentation reference
- Add comments linking to TEST_DATA.md for cross-reference

This alignment makes it easier to verify test failures between
Bruno API tests and Go integration tests using identical credentials.

bruno/user/admin/Register Admin User.bru

@@ -12,10 +12,10 @@ post {
 
 body:json {
   {
-    "email": "admin2@example.com",
-    "username": "admin2",
-    "password": "!Admin@123",
-    "first_name": "Admin",
+    "email": "maxdevices@example.com",
+    "username": "maxdevicesuser",
+    "password": "Test@Pass123!",
+    "first_name": "Test",
     "last_name": "User",
     "role": "admin"
   }
@@ -30,6 +30,10 @@ script:post-response {
   
 }
 
+// Test admin user aligns with Go integration tests
+// Email: maxdevices@example.com used in device cap tests
+// See TEST_DATA.md for shared test data documentation
+
 settings {
   encodeUrl: true
   timeout: 0