dnsimple / dns_erlang / 21822629238

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SVCB params validation (#109)

Fixes #110 and #111. Also adds:

    In SVCB/HTTPS service params, numeric keys 0–6 are reserved and equivalent to the named params (mandatory, alpn, no-default-alpn, port, ipv4hint, ech, ipv6hint). This PR makes the library treat them as such everywhere: name resolution, zone parsing, and JSON parsing all validate key0–key6 the same as their named forms and no longer allow them to be used as generic "unknown" keys.
    Note that this validation was already happening during encoding values back, either to be given on the wire or on the API, so with this change we now validate earlier, at ingestion.

    We also update the documentation generation for json at scripts/generate_json_docs.escript:

    Documented that key0–key6 are equivalent to the named params and are validated the same.
    Clarified no-default-alpn as null and updated the keyNNNN description (e.g. key format and that value is binary or null).

#110

## Summary

The `from_zone/2` function in `dns_svcb_params.erl` has two exception-throwing paths in the unknown-key branch (lines 207-217) that bypass the structured `{error, ...}` return convention used by every other branch in the function.

## Evidence

### 1. Brittle `string:split/3` pattern match (line 209)

```erlang
[KeyNumStr, ""] = string:split(RestStr, "=", leading),
```

If `RestStr` doesn't contain `=`, `string:split/3` returns a single-element list, causing a `badmatch` crash. Similarly, input like `key123=foo=bar` also crashes.

### 2. Unprotected `base64:decode` (line 212)

```erlang
ValueBin = base64:decode(Value),
```

Unlike the `ech=` branch (lines 198-206) which wraps `base64:decode` in a `try/catch` and returns `{error, MakeError(...)}`, the unknown-key branch calls `base64:decode/1` bare. Malformed base64 input raises an unhandled exception.

### 3. Caller doesn't catch exceptions (`dns_zone_decode.erl:1345`)

The caller handles `{ok, _}` and `{error, _}` returns but has no `try/catch`, ... (continued)

60 of 66 new or added lines in 2 files covered. (90.91%)

2617 of 3088 relevant lines covered (84.75%)

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