.Net: Reject mixed-separator UNC paths in file plugins

[jstar0]

Motivation and Context

FileIOPlugin and WebFileDownloadPlugin rejected only some UNC path prefixes before path canonicalization. On Windows, mixed /\ and \/ prefixes resolve to UNC paths too, so those forms could reach filesystem path resolution before the allow-list rejected them.

Fixes #14157.

Description

• Treat any two leading / or \ characters as a UNC or extended-path prefix in both plugins.
• Reject those paths before canonicalization, File.Exists, or File.GetAttributes can probe the filesystem.
• Add read, write, and download regressions for all four separator combinations.

The change is limited to the two plugins named in the issue. Single-separator rooted paths, normal local paths, allow-list behavior, symlink handling, overwrite checks, and download limits are unchanged.

Verification

dotnet test dotnet/src/Plugins/Plugins.UnitTests/Plugins.UnitTests.csproj --filter "FullyQualifiedName~ItRejectsUncOrExtendedPathsOnReadAsync|FullyQualifiedName~ItRejectsUncOrExtendedPathsOnWriteAsync|FullyQualifiedName~DownloadToFileRejectsUncOrExtendedPathsAsync"
dotnet test dotnet/src/Plugins/Plugins.UnitTests/Plugins.UnitTests.csproj --filter "FullyQualifiedName~FileIOPluginTests"
dotnet test dotnet/src/Plugins/Plugins.UnitTests/Plugins.UnitTests.csproj --filter "FullyQualifiedName!~DownloadToFileSucceedsAsync&FullyQualifiedName!~DownloadToFileFailsForInvalidParametersAsync"
dotnet format dotnet/src/Plugins/Plugins.UnitTests/Plugins.UnitTests.csproj --verify-no-changes --no-restore

The focused regressions pass 12/12, all FileIOPluginTests pass 21/21, and the deterministic Plugins unit-test suite passes 409/409. The two excluded Web tests use a live HTTPS request and fail on unmodified main in this macOS environment because certificate revocation status is unavailable.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible (409 deterministic tests pass; two live-network baseline tests are environment-limited as noted above)
• I didn't break anyone 😄

[Copilot]

Pull request overview

Hardens the .NET file-related plugins to consistently reject UNC / extended-path prefixes (including mixed /\ and \/ forms) before path canonicalization can trigger filesystem probes, and adds regression coverage for the newly blocked path-prefix variants.

Changes:

• Update FileIOPlugin and WebFileDownloadPlugin to treat any two leading directory-separator characters (/ or \, in any combination) as a UNC/extended-path prefix and reject early.
• Add unit tests covering all four leading-separator combinations for read/write/download scenarios.
• Align the Web plugin’s per-path allow-list check with the shared UNC/extended prefix detection helper.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
dotnet/src/Plugins/Plugins.Web/WebFileDownloadPlugin.cs	Updates UNC/extended-path detection to reject mixed-separator two-character prefixes before path checks proceed.
dotnet/src/Plugins/Plugins.Core/FileIOPlugin.cs	Applies the same UNC/extended-path prefix rejection to FileIOPlugin path validation.
dotnet/src/Plugins/Plugins.UnitTests/Web/WebFileDownloadPluginTests.cs	Adds download regressions for the newly rejected mixed-separator UNC/extended prefixes.
dotnet/src/Plugins/Plugins.UnitTests/Core/FileIOPluginTests.cs	Adds read/write regressions for the newly rejected mixed-separator UNC/extended prefixes.

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[rogerbarreto]

Fix look good. Reject mixed-separator UNC in all file plugins. Added pre-canonicalization check so relative path that resolve to UNC get stopped before any disk touch. Tests green, format clean.