Provide dig exit code in CouldNotFetchDns exception

[freekmurze]

Why

Discussion #123 reports that when a dig lookup fails, the exception is hard to diagnose:

It makes it seem as if the message ;; IDN output support not enabled was the error, which sent me spiralling trying to debug that message. (...) The exception was thrown because an unsuccessful exit code was returned by dig, but that exit code is not provided in the exception.

CouldNotFetchDns::digReturnedWithError() built its message straight from dig's stderr. dig writes ;; IDN output support not enabled to stderr regardless of success, so when a lookup also failed (typically a transient "no reply from server"), that unrelated warning surfaced as the error message and the real signal, the non-zero exit code, was dropped entirely.

What changed

• The exit code now leads the message and is mapped to dig's documented meaning where known.
• The exit code is exposed via a public ?int $exitCode property so callers can branch on it programmatically.
• dig's stderr is kept as supplementary context rather than masquerading as the cause. The ;; IDN output support not enabled warning can be genuinely useful for internationalized domains, so it is demoted, not discarded.

Message, before and after

Before:

Dig command `...` failed with message: `;; IDN output support not enabled`

After:

Dig command `...` failed with exit code 9 (no reply from server): `;; IDN output support not enabled`

catch (CouldNotFetchDns $exception) {
    $exception->exitCode; // 9
}

The $exitCode is null for the non-dig paths (noHandlerFound, dnsGetRecordReturnedWithError).