Resolve-SnmpCredentialConfig: allow IPv6 CIDR map keys #74

Context

Resolve-SnmpCredentialConfig (or equivalent credential map) only accepts IPv4 CIDR keys, so dual-stack sites cannot scope SNMP creds to IPv6 prefixes after #57/#68.

Tip: c47f0a1.

What

• Allow IPv6 CIDR map keys alongside IPv4.
• Longest-prefix match must work across families without throwing on mixed maps.
• Pester: v6 key match, v4 unchanged, invalid CIDR fails closed.

Acceptance criteria

• IPv6 CIDR keys resolve to the expected credential
• Mixed v4/v6 maps work; wrong-family match returns no credential (not throw)
• Pester coverage for v6 key + longest-prefix

Related

Owl cycle 2 P1#5. Follows #45 longest-prefix, #57/#68.