fix(nxos): parse EVPN and MVPN path columns correctly

[behrang-t]

NX-OS BGP tables can leave Metric and LocPrf blank. The shared EVPN/MVPN parser can then silently interpret Weight and AS-path values as other columns.

For example, a lab EVPN row with Weight 0 and AS path 65003 65002 was parsed as metric 0, localpref 65003 and weight 65002 with the AS path omitted.

This change uses the table header to identify column boundaries and preserve blank numeric fields. It makes localpref optional in the shared schema and updates the five corresponding registry entries. The existing parsing behavior is retained for output without a table header.

The shared implementation affects ShowBgpL2vpnEvpn, ShowBgpIpMvpn and ShowBgpIpMvpnRouteType.

Validation:

• Six focused regression tests passed, covering column combinations, multiple paths, empty output, missing origin and headerless output.
• Golden and empty fixtures passed through folder_parsing_job.py for all three classes.
• Twenty collected EVPN outputs parsed successfully, including 24 rows with the affected AS path.
• The existing ShowBgpL2vpnEvpnRouteType golden fixture passed.

The EVPN fixture comes from lab output; the MVPN fixtures are synthetic coverage for the shared implementation.

make json completed successfully. The registry diff retains only the five relevant schema updates, excluding unrelated regeneration changes.