[Repo Assist] improve: SearchValues in GatewayUrlHelper + 10 SystemCapability tests

[github-actions]

🤖 This is an automated PR from Repo Assist.

Two targeted improvements across Task 5 (Coding) and Task 9 (Testing).

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Task 5 — Coding: SearchValues<char> in GatewayUrlHelper

GatewayUrlHelper.s_authorityTerminators was a char[] passed to string.IndexOfAny(char[], int). This is now a SearchValues<char> used with a span-based IndexOfAny, consistent with the pattern already established in ShellQuoting.cs.

Before:

private static readonly char[] s_authorityTerminators = { '/', '?', '#' };
// ...
var authorityEnd = url.IndexOfAny(s_authorityTerminators, authorityStart);
if (authorityEnd < 0) authorityEnd = url.Length;

After:

private static readonly SearchValues<char> s_authorityTerminators =
    SearchValues.Create("/?#");
// ...
int relativeEnd = url.AsSpan(authorityStart).IndexOfAny(s_authorityTerminators);
var authorityEnd = relativeEnd < 0 ? url.Length : authorityStart + relativeEnd;

SearchValues<char> builds an optimized SIMD lookup structure at startup (SSE2/AVX2 on x64), giving faster character scanning than the legacy overload. The logic is unchanged — same three terminator characters, same semantics.

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Task 9 — Testing: SystemCapability coverage for untested command paths

Three command handlers had zero test coverage: system.run.prepare, system.execApprovals.get, and system.execApprovals.set. The env-sanitization guard in system.run was also untested. Added 10 new tests:

New test	What it verifies
RunPrepare_ReturnsCommandText_ForArray	cmdText field populated from argv array
RunPrepare_ReturnsCommandText_ForString	cmdText uses rawCommand when provided
RunPrepare_ReturnsPlan_WithArgvAndCwd	plan object carries argv, cwd, agentId
RunPrepare_ReturnsError_WhenMissingCommand	Missing command → "Missing command" error
ExecApprovalsGet_WhenNoPolicyConfigured_ReturnsDisabled	enabled: false with no policy set
ExecApprovalsGet_WhenPolicySet_ReturnsRules	enabled: true + rules array when policy wired
ExecApprovalsSet_WhenNoPolicyConfigured_ReturnsError	"No exec policy configured" error
ExecApprovalsSet_UpdatesRules	updated: true, ruleCount: 1 after set
Run_BlockedEnvVar_ReturnsError	PATH env override → blocked error response
Run_AllowedEnvVar_Passes	Custom env var passes through to command runner

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Test Status

• dotnet test OpenClaw.Shared.Tests — 670 passed, 20 skipped (↑10 from 660)
• dotnet test OpenClaw.Tray.Tests — 122 passed (unchanged)

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