Flag conn-fields in hook but absent from provider.yaml in static checks

[dabla]

The check_conn_fields_match_form_widgets static check introduced in #69473
only validated one direction: stale keys declared in conn-fields (YAML) that no
longer exist in get_connection_form_widgets(). It did not catch the opposite
case — a field added to the hook's form widgets but forgotten in conn-fields —
which means the field silently disappears from the new React connection UI.

PR #64643 (Samba auth_protocol) exposed this gap: auth_protocol was added to
get_connection_form_widgets() but not to conn-fields in provider.yaml, and
the check passed without complaint.

The fix makes the check bidirectional: when conn-fields is declared, its keys
must exactly match the hook's widget keys. Both directions are now reported as
errors with distinct fix hints. The "no conn-fields at all" case is still
silently skipped (opting out of the React UI form is intentional).

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Claude Sonnet 4.6

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