[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.

script/check-scan-build

@@ -82,7 +82,7 @@ OT_BUILD_OPTIONS=(
     "-DOT_SRP_SERVER=ON"
     "-DOT_SRP_SERVER_FAST_START_MODE=ON"
     "-DOT_UPTIME=ON"
-    "-DOT_VENDOR_NAME=OpenThread"
+    "-DOT_VENDOR_NAME=RD:OpenThread"
     "-DOT_VENDOR_MODEL=Scan-build"
     "-DOT_VENDOR_SW_VERSION=OT"
 )