dns/ddclient: Improve native Porkbun DDNS support

[jawereq]

Important notices

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• I have read the contributing guidelines at https://github.com/opnsense/plugins/blob/master/CONTRIBUTING.md
• I opened an issue first for non-trivial changes and linked it below.
• AI tools were used to create at least part of the code submitted herewith.

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• Model used: Gemini 3.1 Pro (Extended thinking)
• Extent of AI involvement: Used to help design exception handling and timeout
  logic. Used to review for correctness.

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Describe the problem

Following the recent merge of the initial Porkbun native DDNS client module
(#5599), this pull request introduces several functional enhancements and
network hardening improvements.

While the baseline module works great for simple setups, it currently lacks
automatic record creation and explicit zone boundary handling, which are
critical for users utilizing multi-level subdomains.

I have been running this version on my router for several weeks to update IPv4
and IPv6 wildcard records.

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Describe the proposed solution

This PR adds full lifecycle record management (automatic creation if a record
does not exist), explicit zone support in the Web UI, and robust network
exception handling.

Details

• Dynamic Record Creation: The module now first tries to update an existing
  record using the editByNameType API endpoint (which simplifies the previous
  retrieveByNameType + edit flow). If the edit fails because the record does
  not exist, it automatically falls back to the create endpoint. Users no
  longer need to manually pre-create records on the Porkbun website.
• Explicit Zone Support: Replaced inline domain parsing with dedicated
  _get_zone and _get_label helpers.
• UI Integration: Added service_porkbun to the account.zone field styles
  in dialogAccount.xml so the Web UI exposes the "Zone" field. This allows
  users to explicitly declare zone boundaries for multi-level subdomains (e.g.,
  app.router.example.com), while retaining the fallback heuristic for simple
  domains.
• Network Hardening:
  • Added a global 30-second POST timeout to prevent the backend script from
    hanging.
  • Implemented HTTP 429 rate limit backoff handling using urllib3.util.Retry.
  • Wrapped requests.post() and .json() calls in RequestException and
    JSONDecodeError blocks to prevent network drops from dumping tracebacks
    into syslog.
• Added Python type annotations to all methods to improve code comprehension.