fix(instances): skip IPv6 ExternalIP for Robot servers without a subnet

[Jakob3xD]

Problem

robotNodeAddresses builds the IPv6 ExternalIP of a Robot server by appending 1 to server.ServerIPv6Net (hcloud/instances.go):

if ipv6 {
    // For a given IPv6 network of 2a01:f48:111:4221::, the instance address is 2a01:f48:111:4221::1
    hostAddress := server.ServerIPv6Net + "1"
    addresses = append(addresses, corev1.NodeAddress{Type: corev1.NodeExternalIP, Address: hostAddress})
}

Robot servers do not necessarily have an IPv6 subnet assigned. In that case the Robot API reports an empty server_ipv6_net, the concatenation yields the literal string 1, and that ends up on the Node object:

status:
  addresses:
    - address: "1"
      type: ExternalIP

The same applies to any value that does not yield a valid address after the concatenation — a subnet reported with a prefix length, for example, produces 2001:db8:1234::/641.

This affects HCLOUD_INSTANCES_ADDRESS_FAMILY=ipv6 and dualstack. The default ipv4 never enters this branch.

The equivalent Cloud code path is already guarded (ipv4 && !server.PublicNet.IPv4.IsUnspecified() / ipv6 && !server.PublicNet.IPv6.IsUnspecified()), because it works with real net.IP values instead of string concatenation. The Robot path has no such check.

Impact

The invalid entry is written to status.addresses on every reconciliation and breaks consumers that parse node ExternalIPs. Found on a hybrid cluster whose Robot servers have no IPv6 subnet recorded in Robot.

Fix

• Skip the IPv6 ExternalIP when ServerIPv6Net is empty.
• Validate the concatenated address with net.ParseIP before adding it, and emit an InvalidIPv6Net warning event when the reported subnet does not yield a valid address.

Tests

Three cases added to TestNodeAddressesRobotServer, covering ipv6-only and dual-stack without a subnet, and a malformed subnet. Without the fix they fail as:

--- FAIL: TestNodeAddressesRobotServer/public_ipv6_without_IPv6_subnet
    expected addresses [{Type:Hostname Address:foobar}]
         but got       [{Type:Hostname Address:foobar} {Type:ExternalIP Address:1}]

--- FAIL: TestNodeAddressesRobotServer/public_dual_stack_without_IPv6_subnet
    expected addresses [{Type:Hostname Address:foobar} {Type:ExternalIP Address:203.0.113.7}]
         but got       [{Type:Hostname Address:foobar} {Type:ExternalIP Address:1} {Type:ExternalIP Address:203.0.113.7}]

--- FAIL: TestNodeAddressesRobotServer/public_ipv6_with_malformed_IPv6_subnet
    expected addresses [{Type:Hostname Address:foobar}]
         but got       [{Type:Hostname Address:foobar} {Type:ExternalIP Address:2001:db8:1234::/641}]

[codecov]

Codecov Report

❌ Patch coverage is 83.60656% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.69%. Comparing base (ec8fd02) to head (9528caf).

Files with missing lines	Patch %	Lines
hcloud/instances.go	83.60%	9 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1337      +/-   ##
==========================================
- Coverage   70.11%   66.69%   -3.43%     
==========================================
  Files          29       29              
  Lines        2794     2816      +22     
==========================================
- Hits         1959     1878      -81     
- Misses        643      753     +110     
+ Partials      192      185       -7     

Flag	Coverage Δ
e2e	?
unit	66.69% <83.60%> (+0.19%)	⬆️

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