'keylen' in IPsec is not always an integer

[robindbe]

Describe the bug
When doing a GET on e.g. /api/v2/vpn/ipsec/phase2s it will throw an error 500:

{
  "code": 500,
  "status": "internal server error",
  "response_id": "INTEGER_FIELD_WITH_NON_INTEGER_INTERNAL_VALUE",
  "message": "Cannot parse IntegerField `keylen` from internal because its internal value is not a numeric value. Consider changing this field to a StringField.",
  "data": []
}

I'm pretty sure this happens when one of the phase2's has the key length set to "auto" :

<encryption-algorithm-option>
	<name>aes256gcm</name>
	<keylen>auto</keylen>
</encryption-algorithm-option>

If I set these to 128 bits for example, the GET will work.

To Reproduce
Steps to reproduce the behavior:

1. Configure a VPN with auto key length
2. GET /api/v2/vpn/ipsec/phase2s

pfSense Version & Package Version:

• pfSense Version: 25.11.1
• Package Version: 2.7.6

Affected Endpoints:

• URL: /api/v2/vpn/ipsec/phase2s
  I assume others that use 'keylen'

[jaredhendrickson13]

For the sake of not introducing a breaking change by making this a string field, I've made some changes to allow 0 to be specified as the keylen to represent auto. When this gets written to the config 0 will automatically be converted to auto and vice-versa. I'll have it in the next patch.