.NET: Harden file skill discovery

[SergeyMenshykh]

Motivation & Context

File-based skill discovery should handle filesystem entries consistently and safely across supported platforms. This change aligns the .NET discovery behavior with the corresponding implementation in other languages and makes discovery more robust when entries cannot be safely classified.

Description & Review Guide

• What are the major changes? Adds defensive filesystem-entry validation during file skill discovery, uses fail-closed handling for inspection failures, and adds cross-platform regression coverage.
• What is the impact of these changes? Unsupported or uninspectable discovery entries are skipped while regular skills and explicitly configured roots retain their existing behavior.
• What do you want reviewers to focus on? The discovery boundary behavior and platform-specific test coverage.

Related Issue

N/A — no public issue is linked.

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and the title prefix in sync automatically.

[Copilot]

Pull request overview

This PR hardens .NET file-based skill discovery by treating symlinks/reparse points (and now certain inspection failures) as unsafe during directory traversal, aligning discovery behavior with the intent to avoid escaping configured roots. It also expands unit test coverage for symlink/junction edge cases across platforms.

Changes:

• Add discovery-time validation to skip skill directories/files that are symlinks/reparse points or otherwise unsafe to inspect.
• Update symlink/path inspection logic to treat inspection failures as unsafe (fail-closed) and improve related log messaging.
• Add new unit tests covering symlinked skill directories, symlinked SKILL.md, symlinked configured roots, and Windows junctions.

Reviewed changes

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

File	Description
dotnet/src/Microsoft.Agents.AI/Skills/File/AgentFileSkillsSource.cs	Adds defensive checks during skill discovery and tightens symlink/reparse-point handling + logging.
dotnet/tests/Microsoft.Agents.AI.UnitTests/AgentSkills/FileAgentSkillLoaderTests.cs	Adds regression tests for symlink/junction cases and configured-root behavior.

[github-actions]

Agent Framework Review — Iteration 1

Completed passes: 5 | Result: No high-severity findings

Scope: full PR (1 commit(s)): 59cfbd087aba

Review passes

• Correctness (gpt-5.6-sol) — No issues found in this pass.
• Security Reliability (claude-opus-4.8) — No issues found in this pass.
• Test Coverage (gpt-5.6-sol) — No issues found in this pass.
• Failure Modes (claude-opus-4.8) — No issues found in this pass.
• Design Approach (claude-opus-4.8) — No issues found in this pass.