Add support for route suppression in policy routing by introducing
set suppress-prefix-length for policy route and policy route6 rules.

This maps to Linux ip rule suppress_prefixlength, allowing operators to
reuse main-table routes while suppressing prefixes up to a configured length.

What changed:

• Added new CLI leaf:
  • set suppress-prefix-length <0-128>
  • wired into policy route common rule schema
• Extended policy route apply logic to append:
  • suppress_prefixlength <N> to generated ip rule add ... commands
• Added validation:
  • suppress-prefix-length requires set table or set vrf
  • range constrained by address family:
    • IPv4: 0..32
    • IPv6: 0..128
  • prevent conflicting suppress values for the same resolved table id
• Added smoketests:
  • positive test for IPv4/IPv6 suppress_prefixlength rule presence
  • negative test ensuring commit fails on conflicting values for same table

Files:

• interface-definitions/include/policy/route-common.xml.i
• interface-definitions/include/policy/route-set-suppress-prefix-length.xml.i
• src/conf_mode/policy_route.py
• smoketest/scripts/cli/test_policy_route.py