[netdiag] require 'RD:' prefix for vendor name on reference devices (#12233)

When `OPENTHREAD_CONFIG_REFERENCE_DEVICE_ENABLE` is active, this
change mandates that the vendor name string MUST begin with the "RD:"
prefix. This ensures that reference devices are clearly and
consistently identifiable through network diagnostic queries.

The enforcement is applied at two levels:

- A compile-time `static_assert` is added to validate the default
  `OPENTHREAD_CONFIG_NET_DIAG_VENDOR_NAME` at build time. This uses a
  new `constexpr` helper utility `CheckConstStringPrefix()`.

- A runtime check is added to `otThreadSetVendorName()`, which will
  now return `OT_ERROR_INVALID_ARGS` if an invalid name is provided
  on a reference device build.

All related test configurations (`scan-build`, `toranj`, `nexus`) and
CLI tests are updated to reflect this new requirement and validate
it.

tools/otci/tests/test_otci.py

@@ -353,8 +353,8 @@ class TestOTCI(unittest.TestCase):
         logging.info('dataset active -x: %r', leader.get_dataset_bytes('active'))
         logging.info('dataset pending -x: %r', leader.get_dataset_bytes('pending'))
 
-        leader.set_vendor_name('OpenThread')
-        self.assertEqual('OpenThread', leader.get_vendor_name())
+        leader.set_vendor_name('RD:OpenThread')
+        self.assertEqual('RD:OpenThread', leader.get_vendor_name())
         leader.set_vendor_model('some_model')
         self.assertEqual('some_model', leader.get_vendor_model())
         leader.set_vendor_sw_version('1.0.0')