fix(shell): strip verbatim UNC prefix for Windows terminal and use dunce canonicalize

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Summary

Fixes iOfficeAI/aionrs#270

Problem

On Windows, when clicking 'Open Terminal' for a workspace located on non-system drives (e.g. D:), the system fails with:

指定的路径无效 (The filename, directory name, or volume label syntax is incorrect / path invalid)

Root Cause

1. std::fs::canonicalize() on Windows prepends the \\?\\Extended-Length/Verbatim prefix (e.g. \\?\\D:\workspace).
2. Windows cmd.exe does not support the \\?\\Extended-Length syntax. When executing cmd /c start cmd /K pushd "\\?\\D:\...", cmd.exe rejects the \\?\\... prefix with 指定的路径无效. (In contrast, explorer.exe resolves verbatim paths via Shell APIs, which is why 'Open Folder' worked fine).
3. The rest of the codebase (e.g., in crates/aionui-app/src/services.rs) specifically uses dunce::canonicalize for this exact reason to avoid the \\?\\Extended-Length prefix for Windows CLI compatibility.

Changes

1. Use dunce::canonicalize in crates/aionui-shell/src/shell.rs for validate_file_exists, validate_path_exists, and validate_directory_exists.
2. As defense in depth, sanitize \\?\\UNC\\ and \\?\\ prefixes in build_windows_terminal_command.
3. Add unit test coverage for build_windows_terminal_command verifying verbatim path stripping.