[bug] DataSourceOracle enables DHCPv6 causing IPv6 /128 conflict with IMDS /64

# Bug Report
OCI instances using DataSourceOracle intermittently exhibit IPv6 instability due to conflicting IPv6 configuration sources between IMDS and DHCPv6.

**What appears to be happening**
- IMDS provides a static IPv6 /64 via DataSourceOracle
- cloud-init also enables DHCPv6 on the same interface
- systemd-networkd then assigns a DHCPv6 /128 address
- systemd-networkd rejects/ignores the /128 due to conflict with the existing /64
- the IPv6 /64 lease eventually expires, resulting in temporary loss of IPv6 connectivity
- DHCPv6 later retries and successfully applies the /128, restoring connectivity

On affected instances, systemd-networkd logs show repeated DHCPv6 conflicts:
```
May 04 14:18:11 kajgm-3-00435091 systemd-networkd[4964]: ens300np0: Ignoring DHCPv6 address 2603:c020:401b:4200:0:faa8:95f6:c005/128 (valid for 1d 59min 59s, preferred for 23h 59min 59s) which conflicts with 2603:c020:401b:4200:0:faa8:95f6:c005/64.
```

## Steps to reproduce the problem
1. Launch an OCI instance
2. Observe IPv6 configuration on primary NIC and note the /64 address.
```
ubuntu@kajgm-3-00435091:~$ ip -6 addr show dev ens300np0
2: ens300np0: mtu 1500 qdisc mq state UP group default qlen 1000
altname enp97s0np0
inet6 2603:c020:401b:4200:0:faa8:95f6:c005/64 scope global dynamic noprefixroute
valid_lft 82247sec preferred_lft 78647sec
inet6 fe80::625e:65ff:fed6:44ee/64 scope link
valid_lft forever preferred_lft forever
```
3. And observe the conflicts in the journal logs.
```
ubuntu@kajgm-3-00435091:~$ journalctl -u systemd-networkd | grep -i dhcp
May 04 14:18:11 kajgm-3-00435091 systemd-networkd[4964]: ens300np0: Ignoring DHCPv6 address 2603:c020:401b:4200:0:faa8:95f6:c005/128 (valid for 1d 59min 59s, preferred for 23h 59min 59s) which conflicts with 2603:c020:401b:4200:0:faa8:95f6:c005/64.
```
4. Wait for the /64 address valid_lft to expire.
5. The VM will lose connectivity for a few hours until DHCPv6 retries.
6. Once the VM regains connectivity, SSH back in and observe the primary NIC again - note the /128 address
```
ubuntu@kajgm-3-00435091:~$ ip -6 addr show dev ens300np0
2: ens300np0: mtu 1500 qdisc mq state UP group default qlen 1000
altname enp97s0np0
inet6 2603:c020:401b:4200:0:faa8:95f6:c005/128 scope global dynamic noprefixroute
valid_lft 88681sec preferred_lft 85081sec
inet6 fe80::625e:65ff:fed6:44ee/64 scope link
valid_lft forever preferred_lft forever
```

## Environment details
- Cloud-init version: 25.3-0ubuntu1~24.04.1
- Operating System Distribution: Ubuntu 24.04
- Cloud provider, platform or installer type: Oracle Cloud Infrastructure (OCI)

## cloud-init logs
Unfortunately, collecting full cloud-init logs may not be possible, as the system appears to have a read-only filesystem preventing writes to /home.
```
ubuntu@kajgm-3-00435091:~$ sudo cloud-init collect-logs --redact-sensitive
Unexpected error while running command.
Command: ['tar', 'czf', '/home/ubuntu/cloud-init.tar.gz', '-C', '/run/cloud-init/tmpy37qtgui', 'cloud-init-logs-2026-05-06']
Exit code: 2
Reason: -
Stdout:
Stderr: tar (child): /home/ubuntu/cloud-init.tar.gz: Cannot open: Read-only file system
tar (child): Error is not recoverable: exiting now
tar: /home/ubuntu/cloud-init.tar.gz: Cannot write: Broken pipe
tar: Child returned status 2
tar: Error is not recoverable: exiting now
```