Fix rerendering on invalid form.yml.erb files

[andrejcermak]

This PR aims to fix #5057.

Fix memoisation & UI filtering for malformed Batch Connect apps

• Memoise form_config correctly using defined?(@form_config) so an empty hash is cached and the ERB is not reparsed on every call.
• Cache the result of valid? to avoid repeated form_config look‑ups.
• Filter out apps that fail validation for the saved‑settings dropdown.
• Harden recently_used_apps_helper:
  • safe valid_batch_connect_app? validity check
  • rescue around session context building

[andrejcermak]

Even though this is an individual PR, it could also be seen as part of the caching initiative as it caches invalid apps and doesn't allow re-rendering.

[johrstrom]

The other PR you have open may be eaiser to merge because it's simpler. This, I'm not so sure about. It'll require some thought, which we don't have a lot to spare given we're trying to merge all our accessibility issues and urgently release 4.2 for the same.

[andrejcermak]

@johrstrom thats fine by me, I think the other PR was much more important. This is more of a failsafe mechanism and is not urgent.