fix(network): make address selection deterministic across layers

[iyiguncevik]

This PR fixes non-deterministic address selection by making scope-based
matching deterministic and by clarifying layer responsibilities between network
state and service code.

The issue is that callers often select the first matching address; when
candidate order is unstable, selected addresses can vary across runs.

What changed:

1. domain/network/state/unitinfo.go + domain/network/service/unitinfo.go —no change. This was fixed in fix: filter virtual ethernet devices for relation networks #22054

2. core/network/address.go: Updated SpaceAddresses.AllMatchingScope to return deterministically sorted matches —this is the most important change.

3. domain/network/service/unitaddress.go: GetUnitPrivateAddress now:

• returns first cloud-local match when available;
• otherwise sorts all candidates and returns a deterministic fallback.

4. domain/status/state/model/modelstate.go —no change. It is intentionally left unchanged in this PR because GetMachineFullStatuses currently has downstream expectations on that shape/contract, so “fixing” it here would break expected behavior.

Checklist

• Code style: imports ordered, good names, simple structure, etc
• Comments saying why design decisions were made
• Go unit tests, with comments saying what you're testing
• Integration tests, with comments saying what you're testing
• doc.go added or updated in changed packages

QA steps

Setup network interfaces:

❯ lxc network create lxdbr1 \
  ipv4.address=10.155.6.1/24 \
  ipv4.nat=true \
  ipv6.address=none
❯ lxc network create lxdbr2 \
  ipv4.address=10.155.2.1/24 \
  ipv4.nat=true \
  ipv6.address=none
❯ lxc network create lxdbr3 \
  ipv4.address=10.155.3.1/24 \
  ipv4.nat=true \
  ipv6.address=none
    
❯ lxc network list

❯ lxc profile device add default eth1 nic \
  network=lxdbr1 name=eth1
❯ lxc profile device add default eth2 nic \
  network=lxdbr2 name=eth2
❯ lxc profile device add default eth3 nic \
  network=lxdbr3 name=eth3

❯ lxc profile show default | yq '.devices'
eth0:
  name: eth0
  network: lxdbr0
  type: nic
eth1:
  name: eth1
  network: lxdbr1
  type: nic
eth2:
  name: eth2
  network: lxdbr2
  type: nic
eth3:
  name: eth3
  network: lxdbr3
  type: nic
root:
  path: /
  pool: default
  type: disk

❯ juju bootstrap lxd src

Verify IP addresses

❯ juju show-machine -m controller 0 | yq '.machines["0"].network-interfaces'
eth0:
  ip-addresses:
    - 10.179.172.166/24
    - fd42:4d03:34fb:4185:216:3eff:febc:bb6b/64
  mac-address: 00:16:3e:bc:bb:6b
  gateway: 10.179.172.1
  space: alpha
  is-up: true
eth1:
  ip-addresses:
    - 10.155.6.24/24
  mac-address: 00:16:3e:2f:35:f3
  space: alpha
  is-up: true
eth2:
  ip-addresses:
    - 10.155.2.157/24
  mac-address: 00:16:3e:17:73:e9
  space: alpha
  is-up: true
eth3:
  ip-addresses:
    - 10.155.3.114/24
  mac-address: 00:16:3e:49:12:d9
  space: alpha
  is-up: true

❯ juju add-model m
❯ juju add-ssh-key "$(cat ${JUJU_DATA}/ssh/juju_id_ed25519.pub)"
❯ juju deploy ubuntu-lite qa

❯ juju ssh 0 'ip -4 addr' | grep inet
    inet 127.0.0.1/8 scope host lo
    inet 10.179.172.74/24 metric 100 brd 10.179.172.255 scope global dynamic eth0
    inet 10.155.6.93/24 metric 110 brd 10.155.6.255 scope global dynamic eth1
    inet 10.155.2.190/24 metric 120 brd 10.155.2.255 scope global dynamic eth2
    inet 10.155.3.238/24 metric 130 brd 10.155.3.255 scope global dynamic eth3

❯ juju show-unit qa/0 --format=yaml | yq '."qa/0".public-address'
10.155.2.190

❯ juju exec --unit qa/0 -- unit-get private-address
10.155.2.190                                                  

❯ juju exec --unit qa/0 -- unit-get public-address
10.155.2.190

❯ juju status --format=json | jq -r '.machines["0"]."ip-addresses"[]'
10.155.2.190
10.155.3.238
10.155.6.93
10.179.172.74
fd42:4d03:34fb:4185:216:3eff:fe5e:e9ef

# try 100 times
❯ for i in {1..100}; do juju status --format=json | jq -r '.machines["0"]."ip-addresses"[]'; done | paste - - - - - | sort | uniq -c
    100 10.155.2.190	10.155.3.238	10.155.6.93	10.179.172.74	fd42:4d03:34fb:4185:216:3eff:fe5e:e9ef 

Documentation changes

Links

Issue: Fixes #21809

Jira card: JUJU-9244

[manadart]

One change.

[manadart]

Thanks.

Based on how things are going, this logic is probably on borrowed time, but the change is correct for now.

[nvinuesa]

LGTM, good test coverage, thanks.

[nvinuesa]

As @manadart mentioned, this will probably follow the same kind of implementation as GetUnitPublicAddresses() (i.e. handle the matching/sorting on the state layer with proper SQL).

[iyiguncevik]

/merge