[Repo Assist] fix(canvas): skip GetFinalPathFromHandle containment check on non-Windows

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🤖 This is an automated pull request from Repo Assist.

Summary

Fixes two pre-existing test failures that occurred on every non-Windows CI run:

• CanvasCapabilityTests.A2UIPush_WithJsonlPath_ReadsFile
• A2UICapabilitySecurityTests.A2UIPush_FileJsonl_OverCap_ReturnsError

Root cause

GetFinalPathFromHandle calls the Win32 GetFinalPathNameByHandle API and already guards non-Windows correctly by returning string.Empty (line 440–441):

if (!OperatingSystem.IsWindows())
    return string.Empty;

However the call site unconditionally passed that empty string into IsPathWithinRoot, which resolves it via Path.GetFullPath("") — producing the current working directory — then checked whether CWD falls inside tempRoot. On Linux CI (CWD = /home/runner/work/...), this always evaluated to false, throwing "jsonlPath must resolve within the system temp directory" before any content was read.

Fix

One-line guard: only perform the handle-resolved-path containment check when GetFinalPathFromHandle returns a non-empty string.

// Before
if (!IsPathWithinRoot(finalPath, tempRoot))

// After
if (!string.IsNullOrEmpty(finalPath) && !IsPathWithinRoot(finalPath, tempRoot))

On non-Windows the earlier symlink-resolution check (lines 379–404, using cross-platform FileInfo.ResolveLinkTarget) already provides the equivalent security guarantee.

Security impact

None. The Windows handle-path guard defends against a race where a symlink is swapped between the FileInfo check and the FileStream open. On Linux:

• The FileInfo.ResolveLinkTarget check (lines 379–404) detects symlinks pointing outside temp before the file is opened.
• The OS does not have the same NTFS reparse-point attack surface.

Test Status

Suite	Before	After
OpenClaw.Shared.Tests	❌ 1150 passed / 2 failed / 20 skipped	✅ 1152 passed / 0 failed / 20 skipped
OpenClaw.Tray.Tests	406 passed / 1 pre-existing DPAPI failure (Linux-only, unrelated)	406 passed / 1 pre-existing DPAPI failure (unrelated, unchanged)

./build.ps1 was not run (Linux environment); the fix is a one-line guard in a net10.0 cross-platform library with no platform-specific build requirements.

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