Fix shell working directory on Windows

[xujunfeng1]

What changed

• Build the working_directory shell prefix based on the current OS.
• Keep the existing Unix-style single-quoted cd 'dir' && command behavior for non-Windows systems.
• Use a Windows cmd-compatible cd /d "dir" && command prefix on Windows.
• Added unit coverage for Windows and Unix command-prefix generation, and made the existing working-directory test derive the expected command from the same OS-aware helper.

Why

Fixes #2271. The previous implementation always used Unix single-quote syntax, which cmd.exe does not understand on Windows.

Validation

• Compared main...xujunfeng1:agent/windows-shell-working-directory through the GitHub API and verified only ShellExecuteTool.java and ShellExecuteToolTest.java changed.
• Attempted a sparse shallow clone of xujunfeng1/agentscope-java to run the targeted Maven test, but GitHub HTTPS clone timed out from this environment twice: Failed to connect to github.com port 443 after 21112 ms and 21471 ms.
• Targeted tests were added, but could not be executed locally until the clone/network issue is resolved.

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[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...gentscope/harness/agent/tool/ShellExecuteTool.java	80.00%	0 Missing and 1 partial ⚠️

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