security/acme-client: Fix DNS validation failures - missing prepare() call

[jrork]

Description

This PR fixes a critical bug in the ACME client where DNS validations fail because environment variables (API credentials) are not being passed to acme.sh.

The Problem

The prepare() method in DNS validation classes (like DnsCf.php for Cloudflare) sets environment variables with API credentials:

public function prepare()
{
    $this->acme_env['CF_Token'] = (string)$this->config->dns_cf_token;
    $this->acme_env['CF_Zone_ID'] = (string)$this->config->dns_cf_zone_id;
    // etc...
}

However, the Base::run() method never calls prepare(), so these environment variables are never set, causing DNS validations to fail with exit code 3.

The Solution

Add a call to $this->prepare(); in the run() method before the environment variables are used.

Testing

Tested with Cloudflare DNS validation on OPNsense 25.1:

• Before fix: Validation fails with "domain validation failed (dns01)" and exit code 3
• After fix: Certificate successfully issued

Impact

This bug affects ALL DNS validation providers (Cloudflare, Route53, Azure DNS, etc.) as none of them can pass their credentials to acme.sh.