[Repo Assist] improve: SettingsData.FromJson null safety + GatewayUrlHelper static array

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Summary

Two small, independent coding improvements to SettingsData and GatewayUrlHelper.

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1. SettingsData.FromJson — null/empty input safety

Problem: FromJson declared its parameter as string json (non-nullable). If a caller passes null (e.g. from NRT-unaware code or a test), JsonSerializer.Deserialize throws ArgumentNullException, which is not caught by the existing JsonException handler — the exception would propagate unexpectedly.

Fix: Change the parameter to string? json and return null immediately for null or empty input, consistent with the method's existing contract of returning null for invalid input.

// Before
public static SettingsData? FromJson(string json) { ... }

// After
public static SettingsData? FromJson(string? json)
{
    if (string.IsNullOrEmpty(json))
        return null;
    ...
}

Two regression tests added (null and "" both return null).

---

2. GatewayUrlHelper.RemoveUserInfo — eliminate per-call char array allocation

Problem: RemoveUserInfo contained an inline array literal:

var authorityEnd = url.IndexOfAny(new[] { '/', '?', '#' }, authorityStart);

This allocates a new char[] on the heap every time the method is called. RemoveUserInfo is called from both TryNormalizeWebSocketUrl and SanitizeForDisplay, which are invoked on every connection attempt and every URL-related UI update.

Fix: Extract to a private static readonly field:

private static readonly char[] s_authorityTerminators = { '/', '?', '#' };
// ...
var authorityEnd = url.IndexOfAny(s_authorityTerminators, authorityStart);

The allocation is now paid once at class initialisation rather than once per call.

Test Status

Suite	Before	After
OpenClaw.Shared.Tests	525 passed, 20 skipped	525 passed, 20 skipped (no change)
OpenClaw.Tray.Tests	99 passed	101 passed (+2 new null-safety tests)

All tests pass. No production logic was changed — only defensive input handling and a minor allocation improvement.

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