.NET: [BREAKING] Fix FileSystemJsonCheckpointStore support for sessionIds with path-forbidden or special characters

[lokitoth]

Motivation and Context

The sessionId, an optional parameter when starting a new session when running a workflow is an arbitrary string. This allows consumers to support whatever ids are needed by other systems, but can result in errors when an OS special or forbidden character is included.

Description

The fix is to escape the paths, in a 1:1 manner. We rely on Uri.EscapeDataString to do this. This is a potential breaking change if users created sessionIds containing DataString-encoded characters.

• [BREAKING] Also modifies the index file to make it easier to determine what the name of the file on disk is for a given sessionId.

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

Updates the .NET workflows file-system checkpoint store to safely persist checkpoints when sessionId contains path-forbidden/special characters by encoding the on-disk file name, and adjusts the persisted index format accordingly.

Changes:

• Encode checkpoint file names derived from sessionId/checkpointId to avoid invalid paths and directory traversal.
• Change index.jsonl entries to include both CheckpointInfo and the corresponding on-disk file name.
• Add/extend unit tests covering invalid/path traversal characters in sessionId.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
dotnet/src/Microsoft.Agents.AI.Workflows/Checkpointing/FileSystemJsonCheckpointStore.cs	Escapes checkpoint file names and writes a richer index entry format.
dotnet/src/Microsoft.Agents.AI.Workflows/WorkflowsJsonUtilities.cs	Adds source-gen JSON metadata for the new index entry type.
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/FileSystemJsonCheckpointStoreTests.cs	Adds temp-dir helper and new tests for escaping/path traversal scenarios.

---

You can also share your feedback on Copilot code review. Take the survey.