[Repo Assist] fix(security): remove residual ex.Message leak in canvas jsonlPath error path

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PR #294 sanitized the main canvas command handlers (navigate, eval, snapshot, a2ui.dump, a2ui.caps) but one ex.Message leak was left in the jsonlPath read path in HandleA2UIAsync:

// Before (leaked to gateway)
return Error($"Failed to read jsonlPath: {ex.Message}");

// After (safe)
return Error("Failed to read jsonlPath");

The exception is already written to the local logger on the line immediately above:

Logger.Error($"{request.Command}: failed to read jsonlPath", ex);

So diagnostic fidelity is fully preserved — only the gateway-bound error string is sanitized.

Relation to existing security work

PR	Status	Scope
#291	✅ merged	NodeCapability errors
#294	✅ merged	CanvasCapability + BrowserProxyCapability (most handlers)
#306	🔄 open	WindowsNodeClient, DeviceCapability, approval services
this PR	⬅ here	Residual CanvasCapability jsonlPath handler

Test Status

• dotnet test OpenClaw.Shared.Tests --no-restore — ✅ exit 0
• dotnet test OpenClaw.Tray.Tests --no-restore — ✅ exit 0
• build.ps1 — requires Windows; infrastructure limitation on Linux runner

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