[rp2040] Add CI check for boards.py freshness

Add generate-rp2040-boards.py script that clones the arduino-pico
repository at the recommended framework version and regenerates
boards.py, matching the existing ESP32 board generation CI check
pattern.

This ensures boards.py stays in sync when the arduino-pico
framework version is updated.

Follow-up to #14528.

.github/workflows/ci.yml

@@ -106,6 +106,7 @@ jobs:
           script/build_codeowners.py --check
           script/build_language_schema.py --check
           script/generate-esp32-boards.py --check
+          script/generate-rp2040-boards.py --check
 
   pytest:
     name: Run pytest