odhcp6c: update to 25.12 Git HEAD (2025-12-29)

odhcp6c: update to 25.12 Git HEAD (2025-12-29)

699cc61568b6 dhcpv6: omit IA_NA on Request
8774d3c0ec9c dhcpv6: dhcpv6_send: convert whitespaces to tabs
11abe3790431 ra: convert if block to switch
c05b803d38e9 odhcp6c: do cleanup at exit
6aa4e491a869 config: fix potential memory leaks in error paths
235cdc97d73b all: add log helpers
6e3272d609d3 dhcpv6: clarifying comments
04aea4e3f870 dhcpv6: offload FQDN construction to init_dhcpv6
449ce8374275 dhcpv6: migrate dhcpv6_response_is_valid to switch case

https://github.com/openwrt/odhcp6c/compare/5ab3203875ad...699cc61568b6

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>