fix!: normalise InvalidArgumentError to server_error on the wire

[dhensby]

What

InvalidArgumentError carries the non-standard invalid_argument code but extends OAuthError, so the token and authorize handler catch blocks — which only wrap non-OAuthError errors as ServerError — let it pass straight through and serialize it to the client. This normalises InvalidArgumentError to ServerError at the serialisation boundary, so genuine runtime failures surface as a standard server_error instead of leaking invalid_argument.

Resolves #67.

Why

invalid_argument is defined in none of RFC 6749's error sets (§5.2 token endpoint; §4.1.2.1 / §4.2.2.1 authorization endpoint) nor the IANA registry. Of the ~56 InvalidArgumentError throw sites, ~32 are construction/config-time guards thrown straight back to the integrator that never reach a client — for those, invalid_argument is a useful developer signal and stays as-is. The defect is the small subset thrown during request handling, inside the handler try, which currently serialize as error=invalid_argument / HTTP 500:

• a model returning malformed token data → TokenModel / BearerTokenType throw
• a model returning a falsy authorization code → CodeResponseType throws

Change

Both handler catch blocks now also coerce InvalidArgumentError to ServerError (preserving the original as .inner):

if (!(e instanceof OAuthError) || e instanceof InvalidArgumentError) {
  e = new ServerError(e);
}

Setup-time guards (missing options, model does not implement X(), request/response instance checks) are thrown before the try block, so they're unaffected and still surface InvalidArgumentError to integrator code.

Tests

Added two integration tests proving the wire output for the reachable paths is now server_error/503 (token endpoint) and an error=server_error redirect (authorize endpoint), with the original InvalidArgumentError retained as .inner. Full suite green; no existing InvalidArgumentError assertion changes (all are on pre-try construction / request-instance paths).

BREAKING CHANGE

When a model returns malformed token data or a falsy authorization code at request time, the OAuth error response now reports error: server_error (HTTP 503) instead of error: invalid_argument (HTTP 500). Setup-time InvalidArgumentErrors thrown to integrator code are unchanged.