Fix: Signature mode fallback never retried connect message

[Copilot]

After advancing _signatureTokenMode on a device signature invalid rejection, the client never re-sent the connect message — leaving the WebSocket open but permanently unauthenticated until a full reconnect cycle.

Changes

• New field _challengeNonce — persists the nonce received in connect.challenge so it's available for retries triggered outside the challenge handler
• HandleConnectChallenge — stores _challengeNonce alongside the existing _challengeTimestampMs
• HandleRequestError — after advancing the signature mode, immediately fires SendConnectMessageAsync(_challengeNonce) to retry auth; no retry when all modes are exhausted (mode unchanged)

if (_signatureTokenMode != previousMode)
{
    _logger.Warn($"Gateway rejected device signature with mode {previousMode}; retrying with mode {_signatureTokenMode}");
    _ = SendConnectMessageAsync(_challengeNonce); // was missing — client stuck here
    return;
}

Tests

Four new tests cover: nonce persistence on challenge, per-step mode advancement, connect retry firing on fallback, and full exhaustion of all four modes without a spurious retry.

[shanselman]

Superseded by PR #141 (merged) — signature fallback fix.