.Net: Harden file path validation in Core, Document, and Web plugins

[SergeyMenshykh]

Motivation and Context

This change hardens file system path handling in the file-based plugins. Previously, the path used for allowlist validation could differ from the path ultimately used for file I/O, so the two operations were not guaranteed to refer to the same file system location.

Description

• Adds a shared PathUtilities.GetSafeFullPath helper that produces a canonical full path for a given input.
• Updates FileIOPlugin, SessionsPythonPlugin, DocumentPlugin and WebFileDownloadPlugin to validate and perform I/O against the same canonical path, and to compare paths using OS-appropriate string comparison.
• Adds regression tests covering the new behavior.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines
• All unit tests pass, and I have added new tests where possible

[Copilot]

Pull request overview

This PR hardens filesystem path handling across several .NET file-based plugins by introducing a shared canonicalization helper (PathUtilities.GetSafeFullPath) that resolves paths consistently (including symlink-aware resolution on .NET 6+) so that allowlist validation and subsequent file I/O operate on the same resolved location.

Changes:

• Added PathUtilities.GetSafeFullPath and updated plugins to use it for canonicalization and OS-appropriate path comparisons.
• Hardened allowlist validation logic to reduce validate/use mismatches (including symlink bypass scenarios).
• Added regression tests covering symlink bypass and Linux case-sensitive allowlist behavior; linked PathUtilities.cs into the unit test project.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
dotnet/src/InternalUtilities/src/System/PathUtilities.cs	Introduces shared safe full-path canonicalization with symlink-aware resolution.
dotnet/src/Plugins/Plugins.Core/FileIOPlugin.cs	Uses canonical path for both allowlist checks and I/O; switches to OS-appropriate comparison.
dotnet/src/Plugins/Plugins.Core/CodeInterpreter/SessionsPythonPlugin.cs	Uses safe canonicalization for upload/download validation and directory comparisons.
dotnet/src/Plugins/Plugins.Document/DocumentPlugin.cs	Switches canonicalization and comparisons to PathUtilities to avoid validate/use mismatches.
dotnet/src/Plugins/Plugins.Web/WebFileDownloadPlugin.cs	Canonicalizes download paths via helper and compares paths using PathUtilities.PathComparison.
dotnet/src/Plugins/Plugins.UnitTests/Plugins.UnitTests.csproj	Links PathUtilities.cs into unit tests to validate behavior directly.
dotnet/src/Plugins/Plugins.UnitTests/Core/PathUtilitiesTests.cs	Adds direct tests for canonicalization and symlink handling behaviors.
dotnet/src/Plugins/Plugins.UnitTests/Core/FileIOPluginTests.cs	Adds tests for symlink-bypass denial and Linux case-sensitive allowlist behavior.
dotnet/src/Plugins/Plugins.UnitTests/Core/SessionsPythonPluginTests.cs	Adds tests for symlink-bypass denial and Linux casing behavior in upload/download validation.
dotnet/src/Plugins/Plugins.UnitTests/Web/WebFileDownloadPluginTests.cs	Adds tests for UNC/extended path rejection and Linux case-sensitive allowlist behavior.

Comments suppressed due to low confidence (1)

dotnet/src/Plugins/Plugins.Core/FileIOPlugin.cs:119

• UNC paths can be specified with forward slashes (e.g., "//server/share/file") on Windows. Currently only "\\" is rejected, so "//" can slip through to PathUtilities.GetSafeFullPath / File.Exists before being denied, potentially causing unintended network access or hangs. Reject "//" here as well, consistent with other plugins’ UNC checks.

        if (path.StartsWith("\\\\", StringComparison.OrdinalIgnoreCase))
        {
            throw new ArgumentException("Invalid file path, UNC paths are not supported.", nameof(path));
        }

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