.NET: Harness: Improve path validation

[westey-m]

Motivation and Context

Adding some further improvements and code consolidation to file path validation for the file stores and file based memory store.

Description

Adding some further improvements and code consolidation to file path validation for the file stores and file based memory store.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

This PR strengthens and centralizes relative-path validation for the Harness file stores and the file-based memory provider, aiming to reduce path-traversal risk and normalize path inputs consistently across implementations.

Changes:

• Introduces a shared AgentFileStore.NormalizeRelativePath helper and updates in-memory/file-system stores to use it.
• Updates FileSystemAgentFileStore to normalize before combining with the root directory.
• Adjusts unit tests to reflect new normalization behavior for trailing slashes.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI/Harness/FileMemory/AgentFileStore.cs	Adds centralized relative-path normalization/validation helper.
dotnet/src/Microsoft.Agents.AI/Harness/FileMemory/InMemoryAgentFileStore.cs	Switches to shared normalization helper for all operations.
dotnet/src/Microsoft.Agents.AI/Harness/FileMemory/FileSystemAgentFileStore.cs	Uses normalized relative paths during safe path resolution.
dotnet/src/Microsoft.Agents.AI/Harness/FileMemory/FileMemoryProvider.cs	Refactors and tightens filename validation before combining paths.
dotnet/tests/Microsoft.Agents.AI.UnitTests/Harness/FileMemory/FileSystemAgentFileStoreTests.cs	Updates test expectation: trailing slashes normalize instead of throwing.

Comments suppressed due to low confidence (1)

dotnet/src/Microsoft.Agents.AI/Harness/FileMemory/FileSystemAgentFileStore.cs:250

• The root-escape check uses fullPath.StartsWith(this._rootPath, StringComparison.Ordinal). On Windows, paths are case-insensitive and Path.GetFullPath can normalize drive-letter casing, so Ordinal comparisons can incorrectly reject valid paths (denial of service) depending on the casing of rootDirectory. Use an OS-appropriate comparison (e.g., OrdinalIgnoreCase on Windows) or compare via Path.GetRelativePath(_rootPath, fullPath) and ensure it doesn’t start with ...

        string fullPath = Path.GetFullPath(combined);

        if (!fullPath.StartsWith(this._rootPath, StringComparison.Ordinal))
        {
            throw new ArgumentException(
                $"Invalid path: '{relativePath}'. The resolved path escapes the root directory.",
                nameof(relativePath));