1.4: Keep unknown MAVLink endpoints when downgrading from 1.5

[joaoantoniocardoso]

Summary

• 1.5 persists endpoint types that 1.4 does not know (zenohraw in particular), so pydantic rejected them and ardupilot_manager crashed on downgrade.
• Unknown connection types are now first-class endpoint records: they load and save, show as disabled in the API and UI, and are omitted from the router command line.
• Creating or updating an unknown type returns 422. Deleting one is allowed even if 1.5 marked it protected.

Fixes #4162

Test plan

• On 1.5, persist a 1.5-only endpoint (e.g. zenohraw), then boot this 1.4 build
• ardupilot_manager starts; settings.json still contains the unknown endpoint
• GET /endpoints lists it as disabled; the UI shows the type, with enable/edit off and delete available
• Deleting the unknown endpoint succeeds
• POST/PUT of an unknown type returns 422
• Known endpoints still create, enable, disable, edit, and delete as before