Save changed onboarding connections on the existing agent after validation.
Select the chosen personal provider account and serialize default changes so
an abandoned save cannot override a later choice after Back. Preserve runner,
environment, and unrelated agent settings. Reject unsupported managed-account
to legacy-secret switches with an actionable Connect error.
Verified with 327 component tests, three backend credential-selection tests,
nine affected browser tests, typecheck, build, token gates, all CI jobs, and
Greptile 5/5 with no unresolved comments.
Co-Authored-By: Paperclip <noreply@paperclip.ing>
